Make SubPlan nodes carry the result's typmod as well as datatype OID. This is

Make SubPlan nodes carry the result's typmod as well as datatype OID.  This is
for consistency with the (relatively) recent addition of typmod to SubLink.
An example of why it's a good idea is to be seen in the recent "failed to
locate grouping columns" bug, which wouldn't have happened if a SubPlan
exposed the same typmod info as the SubLink it was derived from.

This could be back-patched, since it doesn't affect any on-disk data format,
but for the moment it doesn't seem necessary to do so.