[repr types] migrate transform::typecheck

[mgree]

A first cut at moving the optimizer over to representation types. My general plan is to migrate everything but the ASTs, starting with transforms and then moving to other parts of the system.

I'm starting with the most type-intensive transform: the typechecker.

Motivation

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

Tips for reviewer

There's some chaff around ScalarBaseType and other fiddly bits that #33557 didn't quite get to... sorry! 🥲

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.