Rollup merge of #82165 - nellshamrell:nell/fix-80658-B, r=estebank

Reword labels on E0308 involving async fn return type

Fix for #80658.

When someone writes code like this:

```rust
fn foo() -> u8 {
    async fn async_fn() -> () {}

    async_fn()
}
```

And they try to compile it, they will see an error that looks like this:

```bash
error[E0308]: mismatched types
 --> test.rs:4:5
  |
1 | fn foo() -> u8 {
  |             -- expected `u8` because of return type
2 |     async fn async_fn() -> () {}
  |                            -- checked the `Output` of this `async fn`, found opaque type
3 |
4 |     async_fn()
  |     ^^^^^^^^^^ expected `u8`, found opaque type
  |
  = note: while checking the return type of this `async fn`
  = note:     expected type `u8`
          found opaque type `impl Future`
```

0 files changed, 0 insertions, 0 deletions