Thread: make ALTER DOMAIN VALIDATE CONSTRAINT no-op when constraint is validated

hi.

Looking at AlterDomainValidateConstraint, it seems currently, ALTER
DOMAIN VALIDATE
CONSTRAINT will re-validate a VALID constraint, which
would just waste cycles.
Ideally, this operation should be a no-op.

The attached patch addresses this by making ALTER DOMAIN VALIDATE CONSTRAINT a
no-op in such cases.

ALTER TABLE VALIDATE CONSTRAINT is already a no-op when the constraint is VALID.