Part#2: Add support for SEMIJOIN/_SEMIJOIN and SUBQUERY hints

(Below are commit comments for several commits that have been squashed into one)

Part#2: Add support for SEMIJOIN/NO_SEMIJOIN hints
        (Parsing of SUBQUERY hint added, but not the functionality)

sql/lex.h
  Added symbols for new optimizer hint keywords
  Fixed some typos in comments

sql/sql_hints.yy
  Added parser rules for query block level hints in general, and specific
  rules for SEMIJOIN, NO_SEMIJOIN, and SUBQUERY hints

sql/parse_tree_hints.h
sql/parse_tree_hints.cc
  Added parse tree class for query block hints and enums for the strategies
  that can be used as arguments to hints

sql/opt_hints.h
sql/opt_hints.cc
  Added pointers to Opt_hints_qb for parse tree classes for
  SEMIJOIN/NO_SEMIJOIN and SUBQUERY hints.
  Added functions semijoin_enabled() and sj_enabled_strategies() to be used to
  compute effects of semijoin hints a query block.

sql/sql_lex.h
sql/sql_lex.cc
  Added private functions semijoin_enabled() and sj_enabled_strategies() to
  st_select_lex class.  If query block level hints are defined for this
  query block, calls are forwarded to corresponding Opt_hints_qb functions.
  Otherwise, return value is based on optimizer_switch setting.

sql/sql_resolver.cc
  In resolve_subquery() call semijoin_enabled() instead of
  checking optimizer_switch directly.
  In convert_subquery_to_semijoin() call sj_enabled_strategies() to determine
  which semi-join strategies may be used.  Stored this information in the
  NESTED_JOIN object of corresponding semijoin nest.

sql/table.h
  Added member sj_enabled_strategies to NESTED_JOIN to be used to store which
  semijoin strategies may be used.

sql/sql_planner.cc
  Change code that checks which semijoin strategies may be used to use
  NESTED_JOIN::sj_enabled_strategies of the corresponding semijoin nest instead
  of optimizer_switch.
  Added optimizer trace for the case where new plan is cheaper but contains
  disabled strategy.  Changed wording of optimizer trace for case when
  a less cheap plan is selected.

sql/sql_optimizer.cc
  Change code that checks which semijoin strategies may be used to use
  NESTED_JOIN::sj_enabled_strategies of the corresponding semijoin nest instead
  of optimizer_switch.

mysql-test/r/opt_hints_subquery.result
mysql-test/t/opt_hints_subquery.test
  Tests for subquery hints

mysql-test/suite/opt_trace/r/subquery_no_prot.result
mysql-test/suite/opt_trace/r/subquery_ps_prot.result
  Changes to optimizer trace (see sql_planner.cc)

WL#8244 Hints for subquery strategies.
Part#2 followup: Fix friend declaration

Prior to c++11, friend declaration of a class needs to contain the "class" word.

WL#8244 Hints for subquery strategies.
Part#3: Add support for SUBQUERY hint

sql/sql_hints.yy
  Changed parser rules for SUBQUERY hint according to WL specifications.
  (It is not possible to list multiple strategies for this hint.)

sql/parse_tree_hints.cc
  PT_qb_level_hint::contextualize()
    Added checks that makes sure that only one SEMIJOIN, NO_SEMIJOIN, and
    SUBQUERY hint is registered per query block.
  PT_qb_level_hint::append_args()
    Added handling of SUBQUERY argument (removed previous dummy implementation)

sql/opt_hints.h
sql/opt_hints.cc
  Added function Opt_hints_qb::subq_strategy() that returns which subquery
  execution strategy to use if SUBQUERY hint is specified.
  Modified Opt_hints_qb::semijoin_enabled() to take SUBQUERY hints into account.

sql/sql_lex.h
sql/sql_lex.cc
  Added function st_select_lex::subq_strategy() that return which subquery
  strategies may be used for the query block
  Added some missing documentation to functions added in Part#2.

sql/sql_optimizer.cc
  Use st_select_lex::subq_strategy() instead of checking optimizer_switch
  settings directly when determining which subquery execution strategy to use.

mysql-test/r/opt_hints_subquery.result
mysql-test/t/opt_hints_subquery.test
  Tests for SUBQUERY hint plus a few additional tests for SEMIJOIN/NO_SEMIJOIN

WL#8244 Hints for subquery strategies.
Fixed a warning

WL#8244 Hints for subquery strategies.
Addendum#1: Fix review comments from Guilhem

sql/sql_hints.yy
  Use same grammar rule for SEMIJOIN/NO_SEMIJOIN hints with and without list
    of strategies.
  Removed Pt_hint_sj::Strategy.  Use OPTIMIZER_SWITCH_* values instead.
  subq => subquery in names

sql/parse_tree_hints.h
  Removed Pt_hint_sj::Strategy.  Use OPTIMIZER_SWITCH_* values instead.
  All enum types shouled have prefix enum_ according to code standard.
  subq => subquery in names
  Removed trailing white space

sql/parse_tree_hints.cc
  Restructured PT_qb_level_hint::contextualize() and
    PT_qb_level_hint::append_args()

sql/opt_hints.h
  subq => subquery in names
  Fixed comments
  Removed trailing white-space

sql/opt_hints.cc
  Restructured Opt_hints_qb::semijoin_enabled() and
    Opt_hints_qb::subquery_strategy()
  Removed trailing white-space

sql/sql_lex.h
  subq => subquery in names
  Removed trailing white-space

sql/sql_lex.cc
  Removed Pt_hint_sj::Strategy.  Use OPTIMIZER_SWITCH_* values instead.
  Restructured st_select_lex::semijoin_enabled()
  subq => subquery in names
  Removed trailing white-space

sql/sql_planner.h
  Changed doc for found_plan_with_allowed_sj

sql/sql_planner.cc
  consider_plan(): check semijoin nest of all tables in DW range for
    allowed strategies, not just the first table.  (In case tables of multiple
    sj nests are interleaved.)
  best_extension_by_limited_search():  No need to check has_sj when deciding
    on pruning.  If there is no semijoin, found_plan_with_allowed_sj will be
    set to true when first plan is found.
  Fix in optimizer_trace:
    plan_use_disabled_strategy => plan_uses_disabled_strategy
  Removed redundant comments
  Removed trailing white-space

sql/sql_optimizer.cc
  subq => subquery in names
  Removed trailing white-space

sql/table.h
  Changed doc for sj_enabled_strategies

mysql-test/t/opt_hints_subquery.test
  Fixed typos in comment statements

mysql-test/r/opt_hints_subquery.result
  Plan changes due to change in consider_plan()
  Fixed typos in comment statements

mysql-test/suite/opt_trace/r/subquery_no_prot.result
mysql-test/suite/opt_trace/r/subquery_ps_prot.result
  Fixed a grammatical error in Optimizer Trace output

WL#8244 Hints for subquery strategies.
Addendum#2: More review comments from Guilhem

sql/sql_hints.yy
  Use Item_exists_subselect::enum_exec_method instead of
    PT_hint_subquery::enum_strategy.

sql/parse_tree_hints.h
  Use Item_exists_subselect::enum_exec_method instead of
    PT_hint_subquery::enum_strategy.
  Renamed flag/flags() to args/get_args() in PT_qb_level_hint

sql/parse_tree_hints.cc
  Renamed flag/flags() to args/get_args() in PT_qb_level_hint
  Updated some comments

sql/opt_hints.h
  Renamed flag/flags() to args/get_args() in PT_qb_level_hint
  Changed get_complex_hints argument from uint to opt_hints_enum
  Some corrections to function docs

sql/opt_hints.cc
  Renamed flag/flags() to args/get_args() in PT_qb_level_hint
  Changed get_complex_hints argument from uint to opt_hints_enum
  Restructured if-tests of Opt_hints_qb::sj_enabled_strategies() and
    Opt_hints_qb::subquery_strategy()

sql/sql_lex.h
  Added doc for opt_hints_qb

sql/sql_lex.cc
  Move initialization of opt_hints_qb to constructor
  Restructured if-tests of st_select_lex::subquery_strategy() and
    st_select_lex::sj_enabled_strategies()

sql/sql_planner.cc
  Updated a comment

sql/sql_resolver.cc
  In setup_tables(): No need to access hints through context.select_lex within
    st_select_lex.

mysql-test/t/opt_hints_subquery.test
mysql-test/r/opt_hints_subquery.result
  Change comment for query to explain why LooseScan can not be used

WL#8244 Hints for subquery strategies.
Cleanup

Reformatted opt_hints_subquery to use uppercase of SQL reserved words.
Removed trailing whitespace

WL#8244 Hints for subquery strategies. Addendum#3

Changes in optimizer_switch after prepare should be reflected in available
semijoin strategies for execution.  (This WL should not change how things
work without hints.)  Hence, enable subquery strategies needs to be recomputed
on every execution instead of during semijoin transformation.

sql/sql_lex.h
sql/sql_lex.cc
  Replaced sj_enabled_strategies() with update_semijoin_strategies().
  The new function iterates over all semijoin nests and recomputes the
  enabled strategies and store result in the corresponding nested_join object.
  Query block level hints are located from first table of semijoin nest since
  original select_lex was lost during semijoin transformation.

sql/sql_optimizer.cc
  Call update_semijoin_strategies() from JOIN::optimize() so that it is
  recomputed on every execution.

sql/sql_resolver.cc
  No longer compute available strategies during semijoin transformation since
  it will be recomputed later anyways.
  setup_tables() should only call adjust_table_hints() on first execution.
  Otherwise, one may adjust hints after semijoin transformation.  This would
  be wrong since tables need to refer to the hints of the original query block.

sql/sql_parse.cc
  Initialize TABLE_LIST hint pointers in st_select_lex::add_table_to_list()

mysql-test/t/opt_hints_subquery.test
mysql-test/r/opt_hints_subquery.result
  Update tests to reflect changed behavior

WL#8244 Hints for subquery strategies. Addendum#4

Addressing review comments from Guilhem on Addendum#3.

sql/sql_lex.cc
  No need to check table pointer.  There is always at least one table per nest
  Fixed comments

sql/sql_lex.h
  Fixed typo

sql/sql_parse.cc
  Removed the added initialization of TABLE_LIST members.  They are
  automatically zeroed by calloc.

 WL#8244 Hints for subquery strategies. Addendum#5.7

Result changes when porting to 5.7 branch.