Fix whole-row Var evaluation to cope with resjunk columns (again).

When a whole-row Var is reading the result of a subquery, we need it to
ignore any "resjunk" columns that the subquery might have evaluated for
GROUP BY or ORDER BY purposes.  We've hacked this area before, in commit
68e4099, but that fix only covered
whole-row Vars of named composite types, not those of RECORD type; and it
was mighty klugy anyway, since it just assumed without checking that any
extra columns in the result must be resjunk.  A proper fix requires getting
hold of the subquery's targetlist so we can actually see which columns are
resjunk (whereupon we can use a JunkFilter to get rid of them).  So bite
the bullet and add some infrastructure to make that possible.

Per report from Andrew Dunstan and additional testing by Merlin Moncure.
Back-patch to all supported branches.  In 8.3, also back-patch commit
292176a, which for some reason I had
not done at the time, but it's a prerequisite for this change.

Author: tglsfdc