(Variant 3, initial variant was by Rex Jonston)

A LEFT JOIN with a constant as a column of the inner table produced wrong
query result if the optimizer had to write the inner table column into a
temp table. Query pattern:

  SELECT ...
  FROM (SELECT /*non-mergeable select*/
        FROM t1 LEFT JOIN (SELECT 'Y' as Val) t2 ON ...) as tbl

Fixed this by adding Item_direct_view_ref::save_in_field() which follows
the pattern of Item_direct_view_ref's save_org_in_field(),
save_in_result_field() and val_XXX() functions:
* call check_null_ref() and handle NULL value
* if we didn't get a NULL-complemented row, call Item_direct_ref's function.