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 https://github.com/rust-lang/rust/issues/143017.