Allow VACUUM FULL/CLUSTER to bump freeze horizons even for pg_class.
pg_class is a special case for CLUSTER and VACUUM FULL, so although
commit
3cff1879f8d03cb729368722ca823a4bf74c0cac caused these
operations to advance relfrozenxid and relminmxid for all other
tables, it did not provide the same benefit for pg_class. This
plugs that gap.
Andres Freund