Auto merge of #17736 - hyf0:hyf_09234908234, r=Veykril

feat(ide-completion): explictly show `async` keyword on `impl trait` methods

OLD:

<img width="676" alt="image" src="https://github.com/user-attachments/assets/f6fa626f-6b6d-4c22-af27-b0755e7a6bf8">

Now:

<img width="684" alt="image" src="https://github.com/user-attachments/assets/efbaac0e-c805-4dd2-859d-3e44b2886dbb">

---

This is an preparation for https://github.com/rust-lang/rust-analyzer/issues/17719.

```rust
use std::future::Future;

trait DesugaredAsyncTrait {
    fn foo(&self) -> impl Future<Output = usize> + Send;
    fn bar(&self) -> impl Future<Output = usize> + Send;
}

struct Foo;

impl DesugaredAsyncTrait for Foo {
    fn foo(&self) -> impl Future<Output = usize> + Send {
        async { 1 }
    }

    //
    async fn bar(&self) -> usize {
        1
    }
}

fn main() {
    let fut = Foo.bar();
    fn _assert_send<T: Send>(_: T) {}
    _assert_send(fut);
}
```

If we don't distinguish `async` or not. It would be confusing to generate sugared version `async fn foo ....` and original form `fn foo`  for `async fn in trait` that is defined in desugar form.

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