This follows in the footsteps of commit 2591ee8ec by removing one more
ill-advised shortcut from planning of GroupingFuncs.  It's true that
we don't intend to execute the argument expression(s) at runtime, but
we still have to process any Vars appearing within them, or we risk
failure at setrefs.c time (or more fundamentally, in EXPLAIN trying
to print such an expression).  Vars in upper plan nodes have to have
referents in the next plan level, whether we ever execute 'em or not.

Per bug #17479 from Michael J. Sullivan.  Back-patch to all supported
branches.

Richard Guo

Discussion: https://postgr.es/m/17479-6260deceaf0ad304@postgresql.org