Fix pg_dump to assign domain array type OIDs during pg_upgrade.

Fix pg_dump to assign domain array type OIDs during pg_upgrade.

During a binary upgrade, all type OIDs are supposed to be assigned by
pg_dump based on their values in the old cluster.  But now that domains
have arrays, there's nothing to base the arrays' type OIDs on, if we're
upgrading from a pre-v11 cluster.  Make pg_dump search for an unused type
OID to use for this purpose.  Per buildfarm.

Discussion: https://postgr.es/m/E1dyLlE-0002gT-H5@gemulon.postgresql.org