sql: string builtins should also be defined for collated strings

[justinj]

Superissue of #20666, there are lots of operations that Postgres supports on collated strings that we don't, for example LIKE (#20666), but also others like LOWER, UPPER, CONCAT, etc.

I'm not sure whether or not it's trivial to define these in terms of their string equivalents - it certainly isn't to do so for the special case of optimizing expressions like LIKE 'abc%', but I think it might be for things like LOWER and UPPER.

cc @knz, @jordanlewis

Jira issue: CRDB-5912

[github-actions]

We have marked this issue as stale because it has been inactive for
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[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!

[knz]

still relevant