Rollup merge of #73672 - nellshamrell:async-fix, r=estebank

Adds a clearer message for when the async keyword is missing from a f…

…unction

This is a somewhat simple fix for #66731.

Under the current version of Rust, if a user has a rust file that looks like this:

```rust
fn boo (){}

async fn foo() {
    boo().await;
}

fn main() {

}
```

And they attempt to run it, they will receive an error message that looks like this:

```bash
error: incorrect use of `await`                                                                                                        --> test.rs:4:14                                                                                                                       |                                                                                                                                   4 |     boo.await();                                                                                                                    |              ^^ help: `await` is not a method call, remove the parentheses                                                                                                                                                                                              error[E0277]: the trait bound `fn() {boo}: std::future::Future` is not satisfied                                                        --> test.rs:4:5                                                                                                                        |                                                                                                                                  4  |     boo.await();                                                                                                                    |     ^^^^^^^^^ the trait `std::future::Future` is not implemented for `fn() {boo}`                                                                                                                                                                                      error: aborting due to 2 previous errors                                                                                                                                                                                                                                    For more information about this error, try `rustc --explain E0277`.
```

This is not very clear.

With the changes made in this PR, when a user compiles and runs that same rust code, they will receive an error message that looks like this:

```bash
error[E0277]: `()` is not a future.
 --> test.rs:4:5
  |
4 |     boo().await;
  |     ^^^^^^^^^^^ `()` is not a future
  |
  = help: the trait `std::future::Future` is not implemented for `()`
  = note: required by `std::future::Future::poll`
```

In the future, I think we should make this error message even clearer, perhaps through a solution like the one described in [this comment](https://github.com/rust-lang/rust/issues/66731#issuecomment-644394287). However, as that potentially involves a major change proposal, I would rather get this change in now and make the error message a little clearer while an MCP is drafted and discussed.

Signed-off-by: Nell Shamrell <nellshamrell@gmail.com>

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