Rollup merge of #136526 - Zalathar:thir-cx, r=Nadrieril

mir_build: Rename `thir::cx::Cx` to `ThirBuildCx` and remove `UserAnnotatedTyHelpers`

A combination of two loosely-related tweaks that would otherwise conflict with each other:

- `Cx` is a pretty unhelpful type name, especially when jumping between THIR-building and MIR-building while trying to make changes to THIR data structures.

- The `UserAnnotatedTyHelpers` trait doesn't appear to provide any benefit over a simple helper function, and its `tcx()` method is currently completely unnecessary.

No functional change.

Author: matthiaskrgr