MDEV-26938 Support descending indexes internally in InnoDB
This is loosely based on the InnoDB changes in
mysql/mysql-server@97fd8b1b6993340b361fa7f85da86a308f0b5e0c
that I had developed in 2015 or 2016.
dict_field_t::descending: A new flag to denote descending order.
cmp_data(), cmp_dfield_dfield(): Add a new parameter descending.
cmp_dtuple_rec(), cmp_dtuple_rec_with_match(): Add a parameter "index".
dtuple_coll_eq(): Replaces dtuple_coll_cmp().
cmp_dfield_dfield_eq_prefix(): Replaces cmp_dfield_dfield_like_prefix().
FIXME: innodb_fts.create fails to return errors because
the HA_REVERSE_SORT flag is not being written to the .frm file
and will be lost in some CREATE TABLE and ALTER TABLE operations.
dict_index_t::is_btree(): Check whether the index is a regular
B-tree index (not SPATIAL, FULLTEXT, or the ibuf.index,
or a corrupted index.
btr_cur_search_to_nth_level_func(): Only attempt to use
the adaptive hash index if index->is_btree().
This function may also be invoked on ibuf.index, and
cmp_dtuple_rec_with_match_bytes() will no longer work on ibuf.index
because it assumes that the index and record fields exactly match.
The ibuf.index is a special variadic index tree.
