Bug#19635706 `!!NEW_CLUSTERED == (INNOBASE_NEED_REBUILD(HA_ALTER_INFO) || ADD_FTS_DOC_ID)'

Problem: Adding a unique index to a NOT NULL POINT column would
trigger a warning and the key would not be promoted. Creating a
unique index on a different NON-NULL column in the same table would
then trigger an assert.

The issue was introduced in the fix for Bug 17665767 which added
special handling of unique indices on NON NULL POINT columns when
reading frm-files, which ensured that such indices were promoted to
primary keys. Since TABLE_SHARE::primary_key is set when reading
the frm file, it was set to the index of the POINT column.

But such columns were not excluded when setting the
HA_KEY_HAS_PART_KEY_SEG bitfield, and since this is used by ALTER
TABLE (sql and innodb) to determine if a key is promotable, the
result was an inconsistency. Specifically, the warning was issued
because TABLE_SHARE::primary_key was different from MAX_KEY when no
key had been promoted.

On the second alter the sql layer trusts TABLE_SHARE::primary_key (via
candidate_key) to imply that the table already has a primary key, and
so does not set Alter_inplace_info::ADD_PK_INDEX. This then triggers
an assert in Innodb which has created a new clustered index, but sees
innobase_need_rebuild(ha_alter_info) returning false (because
Alter_inplace_info::ADD_UNIQUE_INDEX is not covered by the
INNOBASE_ALTER_REBUILD mask, but Alter_inplace_info::ADD_PK_INDEX is).

Solution: Use the algorithm from frm reading also when setting the
HA_KEY_HAS_PART_KEY_SEG bit field, i.e. do not consider keys on POINT
columns as partial.

Note! This is not a good(tm) solution as too many code fragments need
to be kept in sync manually. A better solution would be remove the
dangerous non-persistent HA_KEY_HAS_PART_KEY_SEG bit field and create
an is_key_partial() predicate which could provide this info from
persisted data. Unfortunately, it looks like this approach will
require more refactoring than what is reasonable, now that we're on
the verge of getting a new data dictionary.

(cherry picked from commit 2a3907076385512b1c4dbd8e2c0d21739031c573)