Rollup merge of #91021 - compiler-errors:print_future_output, r=estebank

Elaborate `Future::Output` when printing opaque `impl Future` type

I would love to see the `Output =` type when printing type errors involving opaque `impl Future`.

[Test code](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a800b481edd31575fbcaf5771a9c3678)

Before (cut relevant part of output):
```
note: while checking the return type of the `async fn`
 --> /home/michael/test.rs:5:19
  |
5 | async fn bar() -> usize {
  |                   ^^^^^ checked the `Output` of this `async fn`, found opaque type
  = note:     expected type `usize`
          found opaque type `impl Future`
```

After:
```
note: while checking the return type of the `async fn`
 --> /home/michael/test.rs:5:19
  |
5 | async fn bar() -> usize {
  |                   ^^^^^ checked the `Output` of this `async fn`, found opaque type
  = note:     expected type `usize`
          found opaque type `impl Future<Output = usize>`
```

Note the "found opaque type `impl Future<Output = usize>`" in the new output.

----

Questions:
1. We skip printing the output type when it's a projection, since I have been seeing some types like `impl Future<Output = <[static generator@/home/michael/test.rs:2:11: 2:21] as Generator<ResumeTy>>::Return>` which are not particularly helpful and leak implementation detail.
    * Am I able to normalize this type within `rustc_middle::ty::print::pretty`? Alternatively, can we normalize it when creating the diagnostic? Otherwise, I'm fine with skipping it and falling back to the old output.
    * Should I suppress any other types? I didn't encounter anything other than this generator projection type.
2. Not sure what the formatting of this should be. Do I include spaces in `Output = `?

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