sql: fix FK validation join implementation

[jseldess]

PR: cockroachdb/cockroach#34365

From release notes:

VALIDATE CONSTRAINT is now compatible with the new MATCH FULL and MATCH SIMPLE foreign key semantics and is more performant. [#34365][#34365] {% comment %}doc{% endcomment %}

[jseldess]

@rmloveland, not sure if/how this affects docs.

[rmloveland]

@lucy-zhang ,

Based on my reading of the release note, this appears to be a bugfix/perf thing that does not affect the existing VALIDATE CONSTRAINT docs, since (1) the docs don't mention this specific case (nor its performance), and (2) AFAICT your PR changes the behavior to what a user would expect. Do you agree with that assessment?

[thoszhang]

@rmloveland yes, I agree

[rmloveland]

Thanks! Closing...