Unfortunately ac218aa4 missed the fact that a reference to
'pg_catalog.regnamespace'::regclass wouldn't work before that type is
known. Fix that, by replacing the regtype usage with a join to
pg_type.

Reported-By: Tom Lane
Author: Andres Freund
Discussion: https://postgr.es/m/8863.1543297423@sss.pgh.pa.us
Backpatch: 9.5-, like ac218aa4