Refactor pull_var_clause's API to make it less tedious to extend.

In commit 1d97c19a0f and later c1d9579dd8, we extended
pull_var_clause's API by adding enum-type arguments.  That's sort of a pain
to maintain, though, because it means every time we add a new behavior we
must touch every last one of the call sites, even if there's a reasonable
default behavior that most of them could use.  Let's switch over to using a
bitmask of flags, instead; that seems more maintainable and might save a
nanosecond or two as well.  This commit changes no behavior in itself,
though I'm going to follow it up with one that does add a new behavior.

In passing, remove flatten_tlist(), which has not been used since 9.1
and would otherwise need the same API changes.

Removing these enums means that optimizer/tlist.h no longer needs to
depend on optimizer/var.h.  Changing that caused a number of C files to
need addition of #include "optimizer/var.h" (probably we can thank old
runs of pgrminclude for that); but on balance it seems like a good change
anyway.

src/backend/optimizer/plan/planner.c

@@ -43,6 +43,7 @@
 #include "optimizer/prep.h"
 #include "optimizer/subselect.h"
 #include "optimizer/tlist.h"
+#include "optimizer/var.h"
 #include "parser/analyze.h"
 #include "parser/parsetree.h"
 #include "parser/parse_agg.h"
@@ -3840,7 +3841,7 @@ make_group_input_target(PlannerInfo *root, List *tlist)
 	 * pulled out here, too.
 	 */
 	non_group_vars = pull_var_clause((Node *) non_group_cols,
-									 PVC_RECURSE_AGGREGATES,
+									 PVC_RECURSE_AGGREGATES |
 									 PVC_INCLUDE_PLACEHOLDERS);
 	sub_tlist = add_to_flat_tlist(sub_tlist, non_group_vars);
 
@@ -4088,7 +4089,7 @@ make_window_input_target(PlannerInfo *root,
 	 * at higher levels.
 	 */
 	flattenable_vars = pull_var_clause((Node *) flattenable_cols,
-									   PVC_INCLUDE_AGGREGATES,
+									   PVC_INCLUDE_AGGREGATES |
 									   PVC_INCLUDE_PLACEHOLDERS);
 	new_tlist = add_to_flat_tlist(new_tlist, flattenable_vars);