Don't concatenate binders across types
Partially addresses #83737
There's actually two issues that I uncovered in #83737. The first is that we are concatenating bound vars across types, i.e. in
```
F: Fn(&()) -> &mut (dyn Future<Output = ()> + Unpin)
```
the bound vars on `Future` get set as `for<anon>` since those are the binders on `Fn(&()`. This is obviously wrong, since we should only concatenate directly nested trait refs. This is solved here by introducing a new `TraitRefBoundary` scope, that we put around the "syntactical" trait refs and basically don't allow concatenation across.
Now, this alone *shouldn't* be a super terrible problem. At least not until you consider the other issue, which is a much more elusive and harder to design a "perfect" fix. A repro can be seen in:
```
use core::future::Future;
async fn handle<F>(slf: &F)
where
F: Fn(&()) -> &mut (dyn for<'a> Future<Output = ()> + Unpin),
{
(slf)(&()).await;
}
```
Notice the `for<'a>` around `Future`. Here, `'a` is unused, so the `for<'a>` Binder gets changed to a `for<>` Binder in the generator witness, but the "local decl" still has it. This has heavy intersections with region anonymization and erasing. Luckily, it's not *super* common to find this unique set of circumstances. It only became apparently because of the first issue mentioned here. However, this *is* still a problem, so I'm leaving #83737 open.
r? `@nikomatsakis`
cd56e255c4