Allow defining opaques in check_coroutine_obligations

[compiler-errors]

In the new trait solver, when an obligation stalls on an unresolved coroutine witness, we will stash away the root obligation, even if the stalled obligation is only a distant descendent of the root obligation, since the new solver is purely recursive.

This means that we may need to reprocess alias-relate obligations (and others) which may define opaque types in the new solver. Currently, we use the coroutine's def id as the defining anchor in check_coroutine_obligations, which will allow defining no opaque types, resulting in errors like:

error[E0271]: type mismatch resolving `{coroutine@<source>:6:5: 6:17} <: impl Clone`
 --> <source>:6:5
  |
6 | /     move |_: ()| {
7 | |         let () = yield ();
8 | |     }
  | |_____^ types differ

So this PR fixes the defining anchor and does the same trick as check_opaque_well_formed, where we manually compare opaques that were defined against their hidden types to make sure they weren't defined differently when processing these stalled coroutine obligations.

r? @lcnr cc @cjgillot

[lcnr]

@bors r+

[bors]

📌 Commit 4f958a4 has been approved by lcnr

It is now in the queue for this repository.