ha_innobase::referenced_by_foreign_key(): Protect the check with
dict_sys.freeze(), to prevent races with TRUNCATE TABLE.
The test innodb.instant_alter_crash has been adjusted for this
additional locking.

dict_table_is_referenced_by_foreign_key(): Removed (merged to
the only caller).

create_table_info_t::create_table(): Ignore missing indexes for
FOREIGN KEY constraints if foreign_key_checks=0.

create_table_info_t::create_table_update_dict(): Rewritten as
a static function. Do not return any error.

ha_innobase::create(): When trx!=nullptr and we are operating
on a persistent table, do not rollback, commit, or release the
data dictionary latch.

ha_innobase::truncate(): Protect the entire critical section
with an exclusive dict_sys.latch, so that
ha_innobase::referenced_by_foreign_key() on referenced tables
will return a consistent result. In case of a failure,
invoke dict_load_foreigns() to restore also any FOREIGN KEY
constraints.

ha_innobase::free_foreign_key_create_info(): Define inline.

lock_release(): Disregard innodb_evict_tables_on_commit_debug=ON
when dict_sys.locked() holds. It would hold when fts_load_stopword()
is invoked by create_table_info_t::create_table_update_dict().

dict_sys_t::locked(): Return whether the current thread is holding
the exclusive dict_sys.latch.

dict_sys_t::frozen_not_locked(): Return whether any thread is
holding a shared dict_sys.latch.

In the test main.mysql_upgrade, the InnoDB persistent statistics
will no longer be recalculated in ha_innobase::open() as part of
CHECK TABLE ... FOR UPGRADE. They were deleted earlier in the test.

Tested by: Matthias Leich