Fix handling of COMMENT for domain constraints

Fix handling of COMMENT for domain constraints

For a non-superuser, changing a comment on a domain constraint was
leading to a cache lookup failure as the code tried to perform the
ownership lookup on the constraint OID itself, thinking that it was a
type, but this check needs to happen on the type the domain constraint
relies on.  As the type a domain constraint relies on can be guessed
directly based on the constraint OID, first fetch its type OID and
perform the ownership on it.

This is broken since 7eca575, which has split the handling of comments
for table constraints and domain constraints, so back-patch down to
9.5.

Reported-by: Clemens Ladisch
Author: Daniel Gustafsson, Michael Paquier
Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/15833-808e11904835d26f@postgresql.org
Backpatch-through: 9.5