MDEV-26938 Support descending indexes internally in InnoDB (server part)

* preserve DESC index property in the parser
* store it in the frm (only for HA_KEY_ALG_BTREE)
* read it from the frm
* show it in SHOW CREATE
* skip DESC indexes in opt_range.cc and opt_sum.cc
* ORDER BY test

This includes a fix of MDEV-27432.

mysql-test/main/func_group.test

@@ -206,7 +206,7 @@ create table t1(
   a4 real,
   a5 date,
   key k1(a2,a3),
-  key k2(a4 desc,a1),
+  key k2(a4 /*desc*/,a1),
   key k3(a5,a1)
 );
 create table t2(