Delay creation of subplan tlist until after create_plan().

Once upon a time it was necessary for grouping_planner() to determine
the tlist it wanted from the scan/join plan subtree before it called
query_planner(), because query_planner() would actually make a Plan using
that.  But we refactored things a long time ago to delay construction of
the Plan tree till later, so there's no need to build that tlist until
(and indeed unless) we're ready to plaster it onto the Plan.  The only
thing query_planner() cares about is what Vars are going to be needed for
the tlist, and it can perfectly well get that by looking at the real tlist
rather than some masticated version.

Well, actually, there is one minor glitch in that argument, which is that
make_subplanTargetList also adds Vars appearing only in HAVING to the
tlist it produces.  So now we have to account for HAVING explicitly in
build_base_rel_tlists.  But that just adds a few lines of code, and
I doubt it moves the needle much on processing time; we might be doing
pull_var_clause() twice on the havingQual, but before we had it scanning
dummy tlist entries instead.

This is a very small down payment on rationalizing grouping_planner
enough so it can be refactored.