get_object_address: separate domain constraints from table constraints

Apart from enabling comments on domain constraints, this enables a
future project to replicate object dropping to remote servers: with the
current mechanism there's no way to distinguish between the two types of
constraints, so there's no way to know what to drop.

Also added support for the domain constraint comments in psql's \dd and
pg_dump.

Catalog version bumped due to the change in ObjectType enum.