Better error message for missing return in async fns

[nico-abram]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d8b89e2ba445cca680d18063b9d62a20

async fn f() -> Result<u8, u16> {
    let x = false;
    if x {
        Ok(0u8)
    }
    Err(2u16)
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/lib.rs:4:9
  |
3 | /     if x {
4 | |         Ok(0u8)
  | |         ^^^^^^^ expected `()`, found enum `Result`
5 | |     }
  | |_____- expected this to be `()`
  |
  = note: expected unit type `()`
                  found enum `Result<u8, _>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

Ideally the output should look like it does for the non-async version:

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/lib.rs:4:9
  |
3 | /     if x {
4 | |         Ok(0u8)
  | |         ^^^^^^^ expected `()`, found enum `Result`
5 | |     }
  | |_____- expected this to be `()`
  |
  = note: expected unit type `()`
                  found enum `Result<u8, _>`
help: you might have meant to return this value
  |
4 |         return Ok(0u8);
  |         ++++++        +

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

[BGR360]

@rustbot label +A-async-await +D-papercut

[ThePuzzlemaker]

I'm gonna try and take a shot at this.
@rustbot claim

[eholk]

Thanks for putting up a PR so quickly, @ThePuzzlemaker!

@rustbot label +AsyncAwait-Triaged +AsyncAwait-Polish