pg_upgrade: allow upgrades for new-only TOAST tables

pg_upgrade: allow upgrades for new-only TOAST tables

Previously, when calculations on the need for toast tables changed,
pg_upgrade could not handle cases where the new cluster needed a TOAST
table and the old cluster did not.  (It already handled the opposite
case.)  This fixes the "OID mismatch" error typically generated in this
case.

Backpatch through 9.2