The Item_outer_ref class based on the Item_direct_ref class was always used
to represent an outer field. But if the outer select is a grouping one and the 
outer field isn't under an aggregate function which is aggregated in that
outer select an Item_ref object should be used to represent such a field.
If the outer select in which the outer field is resolved isn't grouping then
the Item_field class should be used to represent such a field.
This logic also should be used for an outer field resolved through its alias
name.

Now the Item_field::fix_outer_field() uses Item_outer_field objects to
represent aliased and non-aliased outer fields for grouping outer selects
only.
Now the fix_inner_refs() function chooses which class to use to access outer
field - the Item_ref or the Item_direct_ref. An object of the chosen class
substitutes the original field in the Item_outer_ref object.
The direct_ref and the found_in_select_list fields were added to the
Item_outer_ref class.


mysql-test/t/subselect3.test:
  Some test cases were corrected after the fix for the bug#27321.
mysql-test/r/subselect3.result:
  Some test cases were corrected after the fix for the bug#27321.
mysql-test/t/subselect.test:
  Added a test case for the bug#27321: Wrong subquery result in a grouping select.
mysql-test/r/subselect.result:
  Added a test case for the bug#27321: Wrong subquery result in a grouping select.
  Some test cases were corrected after this fix.
sql/sql_union.cc:
  Bug#27321: Wrong subquery result in a grouping select.
  Cleanup of the inner_refs_list.
sql/sql_select.cc:
  Bug#27321: Wrong subquery result in a grouping select.
  Now the fix_inner_refs() function chooses which class to use to access outer
  field - the Item_ref or the Item_direct_ref. An object of the chosen class
  substitutes the original field in the Item_outer_ref object.
  A comment is corrected.
sql/item.cc:
  Bug#27321: Wrong subquery result in a grouping select.
  Now the Item_field::fix_outer_field() uses Item_outer_field objects to
  represent aliased and non-aliased outer fields for grouping outer selects
  only.
sql/item.h:
  Bug#27321: Wrong subquery result in a grouping select.
  The direct_ref and the found_in_select_list fields were added to the
  Item_outer_ref class.