Repair logic for reordering grouping sets optimization.

The logic in reorder_grouping_sets to order grouping set elements to
match a pre-specified sort ordering was defective, resulting in
unnecessary sort nodes (though the query output would still be
correct). Repair, simplifying the code a little, and add a test.

Per report from Richard Guo, though I didn't use their patch. Original
bug seems to have been my fault.

Backpatch back to 9.5 where grouping sets were introduced.

Discussion: https://postgr.es/m/CAN_9JTzyjGcUjiBHxLsgqfk7PkdLGXiM=pwM+=ph2LsWw0WO1A@mail.gmail.com

src/test/regress/expected/groupingsets.out

@@ -637,6 +637,19 @@ select a, b, sum(v.x)
    |   |   9
 (12 rows)
 
+-- Test reordering of grouping sets
+explain (costs off)
+select * from gstest1 group by grouping sets((a,b,v),(v)) order by v,b,a;
+                                  QUERY PLAN                                  
+------------------------------------------------------------------------------
+ GroupAggregate
+   Group Key: "*VALUES*".column3, "*VALUES*".column2, "*VALUES*".column1
+   Group Key: "*VALUES*".column3
+   ->  Sort
+         Sort Key: "*VALUES*".column3, "*VALUES*".column2, "*VALUES*".column1
+         ->  Values Scan on "*VALUES*"
+(6 rows)
+
 -- Agg level check. This query should error out.
 select (select grouping(a,b) from gstest2) from gstest2 group by a,b;
 ERROR:  arguments to GROUPING must be grouping expressions of the associated query level

src/test/regress/sql/groupingsets.sql

@@ -213,6 +213,9 @@ select a, b, sum(v.x)
   from (values (1),(2)) v(x), gstest_data(v.x)
  group by cube (a,b) order by a,b;
 
+-- Test reordering of grouping sets
+explain (costs off)
+select * from gstest1 group by grouping sets((a,b,v),(v)) order by v,b,a;
 
 -- Agg level check. This query should error out.
 select (select grouping(a,b) from gstest2) from gstest2 group by a,b;