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.


mysql-test/t/group_by.test:
  Added a test case for the bug#27219: Aggregate functions in ORDER BY.
mysql-test/r/group_by.result:
  Added a test case for the bug#27219: Aggregate functions in ORDER BY.
sql/sql_select.cc:
  Bug#27219: Aggregate functions in ORDER BY.
  Implementation of new check for mixing non aggregated fields and aggregation
  function in the ONLY_FULL_GROUP_BY mode.
sql/sql_lex.cc:
  Bug#27219: Aggregate functions in ORDER BY.
  Initialization of the full_group_by_flag bitmap.
  SELECT_LEX::test_limit function doesn't reset ORDER BY
  clause anymore.
sql/sql_lex.h:
  Bug#27219: Aggregate functions in ORDER BY.
  The full_group_by_flag is added to the SELECT_LEX class.
sql/item_sum.h:
  Bug#27219: Aggregate functions in ORDER BY.
  The outer_fields list is added to the Item_sum class.
sql/mysql_priv.h:
  Bug#27219: Aggregate functions in ORDER BY.
  Defined a set of constants used in the new check for mixing non aggregated
  fields and sum functions in the ONLY_FULL_GROUP_BY_MODE.
sql/item_subselect.cc:
  Bug#27219: Aggregate functions in ORDER BY.
  The Item_in_subselect::select_in_like_transformer function now drops
  ORDER BY clause in all selects in a subquery.
sql/item_sum.cc:
  Bug#27219: Aggregate functions in ORDER BY.
  Now the Item_sum::check_sum_func function now checks whether fields in the
  outer_fields list are aggregated or not and marks selects accordingly.
sql/item.cc:
  Bug#27219: Aggregate functions in ORDER BY.
  Now the Item_field::fix_fields function checks whether the field is aggregated
  or not and marks its select_lex accordingly.