[repr types] Use HIR SQL types in RelationDescs

[mgree]

As MIR moves to representation types (#34351), we must be careful to compute SQL types while we still have HIR. If we don't we run the risk of confusing clients who expected a VARCHAR column and got a TEXT column. (To be clear: the Datums on the wire will always be the same, but the type metadata might be different. BI tools may get confused.)

This PR (with added tests TBD, perhaps at the pgwire level) makes it safe to merge #34350.

Motivation

• This PR adds a known-desirable feature.
  design doc: MIR typechecking using representation types #27239

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.