Fixing grouping expressions inside subqueries

A recent complaint [1] reminded me of an unimplemented case that's
remained unimplemented for something close to 30 years. It's
explained by this comment in parse_agg.c:

* NOTE: we recognize grouping expressions in the main query, but only
* grouping Vars in subqueries. For example, this will be rejected,
* although it could be allowed:
* SELECT
* (SELECT x FROM bar where y = (foo.a + foo.b))
* FROM foo
* GROUP BY a + b;
* The difficulty is the need to account for different sublevels_up.
* This appears to require a whole custom version of equal(), which is
* way more pain than the feature seems worth.

Perhaps I was undercaffeinated when I wrote that, but anyway, there
is a way to do it that doesn't require either a custom version of
equal() or exponential amounts of time to do the matching.
See attached.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/970600119.344834366.1772018068349.JavaMail.zimbra%40meteo.fr