Create stub functions to support pg_upgrade of old contrib/tsearch2.

Commits 9ff60273e3 and dbe2328959 adjusted the declarations
of some core functions referenced by contrib/tsearch2's install script,
forgetting that in a pg_upgrade situation, we'll be trying to restore
operator class definitions that reference the old signatures.  We've
hit this problem before; solve it in the same way as before, namely by
installing stub functions that have the expected signature and just
invoke the correct function.  Per report from Jeff Janes.

(Someday we ought to stop supporting contrib/tsearch2, but I'm not
sure today is that day.)

src/backend/utils/adt/tsquery_gist.c

@@ -262,3 +262,16 @@ gtsquery_picksplit(PG_FUNCTION_ARGS)
 
 	PG_RETURN_POINTER(v);
 }
+
+/*
+ * Formerly, gtsquery_consistent was declared in pg_proc.h with arguments
+ * that did not match the documented conventions for GiST support functions.
+ * We fixed that, but we still need a pg_proc entry with the old signature
+ * to support reloading pre-9.6 contrib/tsearch2 opclass declarations.
+ * This compatibility function should go away eventually.
+ */
+Datum
+gtsquery_consistent_oldsig(PG_FUNCTION_ARGS)
+{
+	return gtsquery_consistent(fcinfo);
+}