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Context-independent parsing of the SELECT statement.

WL#7203: True bottom-up server parser: refactoring of the SET statement
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Context-independent parsing of the SET statement.

WL#7199: True bottom-up server parser: common framework for the refactoring
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New files:

  sql/parse_location.h
    A separate definition of YYLTYPE structure (Bison "locations")
    and its Bison macros: we don't need the whole sql_lex.h file to
    include this structure -- this helps to break cyclic include
    file dependences.

  sql/parse_tree_node_base.h
  sql/parse_tree_node_base.cc
    Definition of parse tree node base class: the
    Parse_tree_node class.

  sql/parse_tree_helpers.h
  sql/parse_tree_helpers.cc
    Helper classes and macros:
    * The Parse_tree_item: the child of the Item class that overload
      abstract methods of its parent; we subclass it with intermediate
      Item-compatible parse tree node classes.
    * Helper macros for recursive Item list rule refactoring.

  sql/parse_tree_items.h
  sql/parse_tree_items.cc
    Files to store Item-compatible parse tree node classes.

  sql/parse_tree_nodes.h
  sql/parse_tree_nodes.cc
    Files to store parse tree node classes (besides Item-compatible
    ones).

Other changes:

* The LEX::text_string_is_7bit has replaced with the
  Lex_input_stream::text_string_is_7bit() constant function.
* sql_yacc.yy: the %union has been moved to sql_lex.h and the
  MYSQL_YACC-related hacks have been eliminated.
* YYLTYPE has been structured with the help of new Symbol_location class