MDEV-10122: MariaDB does not support group functions in some contexts where MySQL does

The problematic queries involve unions. For unions we have an
optimization where we skip the ORDER BY clause in a query from one side
of the union if it will be performed later due to UNION.
EX:
(SELECT a from t1 ORDER BY a) ORDER BY b;
The first ordering by a is not necessary and it gets removed.

The problem is that we still need to resolve the Items before removing the
ORDER BY list from the
SELECT_LEX structure. During this final resolve step however, we forgot to
allow SET functions within the ORDER BY clause. This caused us to return
an "Invalid use of group function" error during the checking performed
by fix_fields in Item_sum::init_sum_func_check.

mysql-test/r/parser.result

@@ -951,13 +951,23 @@ a
 10
 20
 SELECT 1 AS a UNION (SELECT a FROM t1 GROUP BY a ORDER BY GROUP_CONCAT(a));
-ERROR HY000: Invalid use of group function
+a
+1
+10
+20
+30
 SELECT 1 AS a UNION (SELECT a FROM t1 GROUP BY a ORDER BY GROUP_CONCAT(a)) LIMIT 1;
-ERROR HY000: Invalid use of group function
+a
+1
 SELECT 1 AS a UNION (SELECT a FROM t1 GROUP BY a ORDER BY GROUP_CONCAT(a)) ORDER BY a;
-ERROR HY000: Invalid use of group function
+a
+1
+10
+20
+30
 SELECT 1 AS a UNION (SELECT a FROM t1 GROUP BY a ORDER BY GROUP_CONCAT(a)) ORDER BY a LIMIT 1;
-ERROR HY000: Invalid use of group function
+a
+1
 DROP TABLE t1;
 # UNION with a parenthesized term with ROLLUP
 CREATE TABLE t1 (a INT);