Auto merge of #83870 - jackh726:binder-refactor-fix, r=nikomatsakis

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`

compiler/rustc_mir/src/transform/generator.rs

@@ -751,9 +751,10 @@ fn sanitize_witness<'tcx>(
             span_bug!(
                 body.span,
                 "Broken MIR: generator contains type {} in MIR, \
-                       but typeck only knows about {}",
-                decl.ty,
-                witness,
+                       but typeck only knows about {} and {:?}",
+                decl_ty,
+                allowed,
+                allowed_upvars
             );
         }
     }