Constant expressions that appear in ORDER BY, GROUP BY, DISTINCT ON

Constant expressions that appear in ORDER BY, GROUP BY, DISTINCT ON
lists should be reverse-compiled into targetlist index numbers, because
that's the only interpretation the parser allows for a constant in these
clauses.  (Ergo, the only way they could have gotten into the list in
the first place is to have come from the targetlist; so this should always
work.)  Per problem report from Peter E.