pg_upgrade: prevent oid conflicts with new-cluster TOAST tables

Previously, TOAST tables only required in the new cluster could cause
oid conflicts if they were auto-numbered and a later conflicting oid had
to be assigned.

Backpatch through 9.3

contrib/pg_upgrade/pg_upgrade.h

@@ -335,6 +335,7 @@ void		disable_old_cluster(void);
 /* dump.c */
 
 void		generate_old_dump(void);
+void		optionally_create_toast_tables(void);
 
 
 /* exec.c */