subquery output column exactly once left-to-right.  Although this is the case
in the original parser output, it might not be so after rewriting and
constant-folding, as illustrated by bug #3882 from Jan Mate.  Instead
scan the subquery's target list to obtain needed per-column information;
this is duplicative of what the parser did, but only a couple dozen lines
need be copied, and we can clean up a couple of notational uglinesses.
Bug was introduced in 8.2 as part of revision of SubLink representation.