Support partial UniqueWithoutIndex referencing types

[Xiang-Gu]

Today, we didn't correctly support UniqueWithoutIndex constraint that references another user-defined
type, both in the legacy and declarative schema changer.

Namely, if we add the unique_without_index constraint as follows:

CREATE TYPE typ AS ENUM ('a', 'b');
CREATE TABLE t (i INT PRIMARY KEY, j STRING);
SET experimental_enable_unique_without_index_constraints = true;
ALTER TABLE t ADD UNIQUE WITHOUT INDEX (j) WHERE (j::typ != 'a');

Then we inspect the type descriptor

SELECT jsonb_pretty(crdb_internal.pb_to_json('cockroach.sql.sqlbase.Descriptor', descriptor) ) FROM system.descriptor WHERE id = 112;

we will see there is no back-reference from this type descriptor typ back to table t.

This happens both in the legacy and declarative schema changer.

Jira issue: CRDB-24273

[Xiang-Gu]

Note: I didn't mention "UWI referencing sequences" because that's not implemented/allowed (and probably will never be supported/allowed). See here. We will thus only address the issue of a UWI referencing types.