Bug#27219: Aggregate functions in ORDER BY.

Mixing aggregate functions and non-grouping columns is not allowed in the
ONLY_FULL_GROUP_BY mode. However in some cases the error wasn't thrown because
of insufficient check.

In order to check more thoroughly the new algorithm employs a list of outer
fields used in a sum function and a SELECT_LEX::full_group_by_flag.
Each non-outer field checked to find out whether it's aggregated or not and
the current select is marked accordingly.
All outer fields that are used under an aggregate function are added to the
Item_sum::outer_fields list and later checked by the Item_sum::check_sum_func
function.

sql/mysql_priv.h

@@ -1044,6 +1044,13 @@ SQL_SELECT *make_select(TABLE *head, table_map const_tables,
                         bool allow_null_cond,  int *error);
 extern Item **not_found_item;
 
+/*
+  A set of constants used for checking non aggregated fields and sum
+  functions mixture in the ONLY_FULL_GROUP_BY_MODE.
+*/
+#define NON_AGG_FIELD_USED  1
+#define SUM_FUNC_USED       2
+
 /*
   This enumeration type is used only by the function find_item_in_list
   to return the info on how an item has been resolved against a list