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/commands/trigger.c

@@ -352,9 +352,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 		 * subselects in WHEN clauses; it would fail to examine the contents
 		 * of subselects.
 		 */
-		varList = pull_var_clause(whenClause,
-								  PVC_REJECT_AGGREGATES,
-								  PVC_REJECT_PLACEHOLDERS);
+		varList = pull_var_clause(whenClause, 0);
 		foreach(lc, varList)
 		{
 			Var		   *var = (Var *) lfirst(lc);