Rollup merge of #128580 - compiler-errors:cfi-param-env, r=lcnr

Use `ParamEnv::reveal_all` in CFI

I left a huge comment for why this ICEs in the test I committed.

`typeid_for_instance` should only be called on monomorphic instances during codegen, and we should just be using `ParamEnv::reveal_all()` rather than the param-env of the instance itself. I added an assertion to ensure that we only do this for fully substituted instances (this may break with polymorphization, but I kinda don't care lol).

Fixes #114160
cc `@rcvalle`

Author: matthiaskrgr