PG16: Reset PlannerInfo->placeholdersFrozen

Reset the flag to control PlaceHolderInfo creation because we're copying
the entire content of the (PlannerInfo *)root data structure when
building the first/last path.

postgres/postgres@b3ff6c74

src/planner/agg_bookend.c

@@ -585,6 +585,11 @@ build_first_last_path(PlannerInfo *root, FirstLastAggInfo *fl_info, Oid eqop, Oi
 	subroot->parse = parse = copyObject(root->parse);
 	IncrementVarSublevelsUp((Node *) parse, 1, 1);
 
+#if PG16_GE
+	/* Reset placeholdersFrozen: https://github.com/postgres/postgres/commit/b3ff6c74 */
+	subroot->placeholdersFrozen = false;
+#endif
+
 	/* append_rel_list might contain outer Vars? */
 	subroot->append_rel_list = copyObject(root->append_rel_list);
 	IncrementVarSublevelsUp((Node *) subroot->append_rel_list, 1, 1);