Improve generated column names for cases involving sub-SELECTs.

Improve generated column names for cases involving sub-SELECTs.

We'll now use "exists" for EXISTS(SELECT ...), "array" for ARRAY(SELECT
...), or the sub-select's own result column name for a simple expression
sub-select.  Previously, you usually got "?column?" in such cases.

Marti Raudsepp, reviewed by Kyotaro Horiugchi