Re: Wrong results with grouping sets

If the grouping expression is a Var or PHV, we can just set its
nullingrels, very straightforward.   For an expression that is neither a
Var nor a PHV, I'm not quite sure how to set the nullingrels.  I tried
the idea of wrapping it in a new PHV to carry the nullingrels, but that
may cause some unnecessary plan diffs.  In the patch for such an
expression I just set the nullingrels of Vars or PHVs that are contained
in it.  This is not really 'correct' in theory, because it is the whole
expression that can be nullable by grouping sets, not its individual
vars.  But it works in practice, because what we need is that the
expression can be somehow distinguished from the same expression in ECs,
and marking its vars is sufficient for this purpose.  But what if the
expression is variable-free?  This is the point that needs more work.
Fow now the patch just handles variable-free expressions of type Const,
by wrapping it in a new PHV.