sql: drop and add constraint with same name in one statement

[Xiang-Gu]

The following problem exists in legacy schema changer:

Setup:

CREATE TABLE t (i INT PRIMARY KEY);
ALTER TABLE t ADD CONSTRAINT check_i CHECK (i > 0);

Troubled Stmt:

ALTER TABLE t DROP CONSTRAINT check_i, ADD CONSTRAINT check_i CHECK (i > 0), VALIDATE CONSTRAINT check_i;

(The first easy-to-fix error is the ADD CONSTRAINT will complain that the name "check_i" is already used. This is easily fixed by adding some logic in alter_table.go:validateConstraintNameIsNotUsed)

After that, it still errors with the VALIDATE CONSTRAINT clause saying "constraint check_i does not exist". The root cause is that we deduplicated when we build the constraint cache so that catalog.FindConstraintByName in VALIDATE CONSTRAINT will see the dropping constraint check_i, and fail to realize there is another adding constraint (with the same name).

I don't have an idea on how to fix it elegantly, and it's probably not worth it since this can be beautifully handled in the declarative schema changer, but I still open this issue for tracking.

Jira issue: CRDB-24354

[Xiang-Gu]

#96648 is the PR that allows the declarative schema changer to handle this case correctly.