wrong results
 Mark the containing Item(s) (Item_subselect descendant usually) of 
 a subselect as containing aggregate functions if it has references
 to aggregates functions that are calculated outside its context.
 This tels end_send_group() not to make an Item_subselect descendant in
 select list a copy and causes the correct value being returned.


mysql-test/r/func_group.result:
  Bug #21540: Subqueries with no from and aggregate functions return
              wrong results
   - test cases
mysql-test/r/subselect.result:
  Bug #21540: Subqueries with no from and aggregate functions return
              wrong results
   - fixed the result of an existing testcase.
mysql-test/t/subselect.test:
  Bug #21540: Subqueries with no from and aggregate functions return
              wrong results
   - test cases
sql/item_sum.cc:
  Bug #21540: Subqueries with no from and aggregate functions return
              wrong results
   Mark the containing Item (Item_subselect descendant usually) of 
   a subselect as containing aggregate functions if it has references
   to aggregates functions that are calculated outside its context.
   This tels end_send_group() not to make an Item_subselect descendant in
   select list a copy and causes the correct value being returned.