INCORRECT RESULTS IN 5.6

Issue
-----
This issue occurs with a query of the form
SQ1 IN (SQ2 IN (SQ3)). To handle the first IN, a semijoin
is used and the subquery is materialized. The second IN
is converted to EXISTS optimization.

During IN-TO-EXISTS transform for SQ3,
Item_in_subselect's used_tables_cache is set to 3 instead
of 2. This is because <left expr> is a direct_ref to
concat(a,''). It visits Item_ref::resolved_used_tables.
The used_tables_cache of concat(a,'') is set to 1. This
difference allows for the condition to be eliminated by
make_cond_for_table_from_pred, since the table, represented
by used_tables_cache, is not referenced.


SOLUTION:
---------
This is the same problem fixed in Bug#21139722.

The issue is solved by creating a new used_tables_for_level
for Item_field class. This will return the table number if
the column is resolved in the current query block. If it is
resolved in an outer query block, the appropriate value is
returned.

Since constant tables are not handled in
Item_field::used_tables_for_level, Bug#21139402 is also
backported.