Auto merge of #141762 - compiler-errors:witnesser, r=lcnr

Unify `CoroutineWitness` sooner in typeck, and stall coroutine obligations based off of `TypingEnv`

* Stall coroutine obligations based off of `TypingMode` in the old solver.
* Eagerly assign `TyKind::CoroutineWitness` to the witness arg of coroutines during typeck, rather than deferring them to the end of typeck.

r? lcnr

This is part of rust-lang/rust#143017.

Author: bors