Bug#23800: Outer fields in correlated subqueries is used in a temporary table

created for sorting.

Any outer reference in a subquery was represented by an Item_field object.
If the outer select employs a temporary table all such fields should be
replaced with fields from that temporary table in order to point to the 
actual data. This replacement wasn't done and that resulted in a wrong
subquery evaluation and a wrong result of the whole query.

Now any outer field is represented by two objects - Item_field placed in the
outer select and Item_outer_ref in the subquery. Item_field object is
processed as a normal field and the reference to it is saved in the
ref_pointer_array. Thus the Item_outer_ref is always references the correct
field. The original field is substituted for a reference in the
Item_field::fix_outer_field() function.

New function called fix_inner_refs() is added to fix fields referenced from
inner selects and to fix references (Item_ref objects) to these fields.

The new Item_outer_ref class is a descendant of the Item_direct_ref class.
It additionally stores a reference to the original field and designed to
behave more like a field.

sql/sql_lex.cc

@@ -1192,6 +1192,7 @@ void st_select_lex::init_select()
   is_correlated= 0;
   cur_pos_in_select_list= UNDEF_POS;
   non_agg_fields.empty();
+  inner_refs_list.empty();
 }
 
 /*