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/util/placeholder.c

@@ -220,7 +220,7 @@ find_placeholders_in_expr(PlannerInfo *root, Node *expr)
 	 * convenient to use.
 	 */
 	vars = pull_var_clause(expr,
-						   PVC_RECURSE_AGGREGATES,
+						   PVC_RECURSE_AGGREGATES |
 						   PVC_INCLUDE_PLACEHOLDERS);
 	foreach(vl, vars)
 	{
@@ -354,7 +354,7 @@ fix_placeholder_input_needed_levels(PlannerInfo *root)
 	{
 		PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
 		List	   *vars = pull_var_clause((Node *) phinfo->ph_var->phexpr,
-										   PVC_RECURSE_AGGREGATES,
+										   PVC_RECURSE_AGGREGATES |
 										   PVC_INCLUDE_PLACEHOLDERS);
 
 		add_vars_to_targetlist(root, vars, phinfo->ph_eval_at, false);