Some assorted region hashing fixes.
This PR contains three changes.
1. It changes what we implement `HashStable` for. Previously, the trait was implemented for things in the local `TyCtxt`. That was OK, since we only invoked hashing with a `TyCtxt<'_, 'tcx, 'tcx>` where there is no difference. With query result hashing this becomes a problem though. So we now implement `HashStable` for things in `'gcx`.
2. The PR makes the regular `HashStable` implementation *not* anonymize late-bound regions anymore. It's a waste of computing resources and it's not clear that it would always be correct to do so.
3. The PR adds an option for stable hashing to treat all regions as erased and uses this new option when computing the `TypeId`. This should help with https://github.com/rust-lang/rust/issues/41875.
I did not add a test case for (3) since that's not possible yet. But it looks like @zackmdavis has something in the pipeline there `:)`.
r? @eddyb
73c3f55a3e