WL#3487: Subquery optimization: Correct evaluation of subqueries

Affects: Server-5.0   —   Status: Complete   —   Priority: Low

Make all kinds of subqueries to be evaluated correctly. Currently the only
remaining known problem is BUG#8804: execution of "outer_expr NOT IN (SELECT ...)"

The result is incorrect when the outer_expr is NULL and (SELECT ...) produces an
empty set.

Per Igor:
This feature was implemented by the fix for
BUG#8804 "wrong result for [null_expr] IN (SELECT ...)",
pushed to 5.0.32 and 5.1.14. 
 
Fix for BUG#8804 "wrong result for [null_expr] IN (SELECT ...)".
No further HLS needed.
Fix for BUG#8804 "wrong result for [null_expr] IN (SELECT ...)".
No further LLD needed.