Fix ALTER TABLE code to update domain constraints when needed.

Fix ALTER TABLE code to update domain constraints when needed.

It's possible for dropping a column, or altering its type, to require
changes in domain CHECK constraint expressions; but the code was
previously only expecting to find dependent table CHECK constraints.
Make the necessary adjustments.

This is a fairly old oversight, but it's a lot easier to encounter
the problem in the context of domains over composite types than it
was before.  Given the lack of field complaints, I'm not going to
bother with a back-patch, though I'd be willing to reconsider that
decision if someone does complain.

Patch by me, reviewed by Michael Paquier

Discussion: https://postgr.es/m/30656.1509128130@sss.pgh.pa.us