LEFT JOIN(SUBQUERY) AND ORDER BY


Issue:
------
This bug can occur when:
- there is more than one inner tables in the join nest.
- an Item_direct_view_ref object references a column
  from a const (system) table.

This has caused the following regressions :
Bug# 26596977, Bug# 26616066 and Bug# 26618455

Fix for 8.0:
-----------
When the object is of type FIELD_ITEM (i.e. belongs to a
table), Item_direct_view_ref::used_tables should return the
related table's map. In other cases it returns the first
inner table's map.

Fix for 5.7:
------------
The function Item_direct_view_ref::used_tables has been
removed and the fix will limited to handle the problem in
temporary tables.

While creating fields for the temporary table, consider
field as constant only if:
1) The item is constant and
   1a) Isn't part of a derived table (or view). OR
   1b) The item belongs to a derived table (or view) and it
       doesn't belong to an inner table of an outer join.