Rollup merge of #135492 - metamuffin:bug-invalid-await-suggest, r=compiler-errors

Add missing check for async body when suggesting await on futures.

Currently the compiler suggests adding `.await` to resolve some type conflicts without checking if the conflict happens in an async context. This can lead to the compiler suggesting `.await` in function signatures where it is invalid. Example:

```rs
trait A {
    fn a() -> impl Future<Output = ()>;
}
struct B;
impl A for B {
    fn a() -> impl Future<Output = impl Future<Output = ()>> {
        async { async { () } }
    }
}
```
```
error[E0271]: expected `impl Future<Output = impl Future<Output = ()>>` to be a future that resolves to `()`, but it resolves to `impl Future<Output = ()>`
 --> bug.rs:6:15
  |
6 |     fn a() -> impl Future<Output = impl Future<Output = ()>> {
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found future
  |
note: calling an async function returns a future
 --> bug.rs:6:15
  |
6 |     fn a() -> impl Future<Output = impl Future<Output = ()>> {
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `A::{synthetic#0}`
 --> bug.rs:2:27
  |
2 |     fn a() -> impl Future<Output = ()>;
  |                           ^^^^^^^^^^^ required by this bound in `A::{synthetic#0}`
help: consider `await`ing on the `Future`
  |
6 |     fn a() -> impl Future<Output = impl Future<Output = ()>>.await {
  |                                                             ++++++
```

The documentation of suggest_await_on_expect_found (`compiler/rustc_trait_selection/src/error_reporting/infer/suggest.rs:156`) even mentions such a check but does not actually implement it.

This PR adds that check to ensure `.await` is only suggested within async blocks.

There were 3 unit tests whose expected output needed to be changed because they had the suggestion outside of async. One of them (`tests/ui/async-await/dont-suggest-missing-await.rs`) actually tests that exact problem but expects it to be present.

Thanks to `@llenck` for initially noticing the bug and helping with fixing it

Author: matthiaskrgr