When generating implementation for a trait, emit `async fn` for functions that return `impl Future (+ Send)`

With AFITs + RPITITs being stabilized in 1.75 without RTN, the following pattern will probably become common:

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

impl AlwaysSend for SomeType {
    // We can use `async fn` syntax here, yay!
    async fn foo(&self) { /* ... */ }
}

Unfortunately, the “fill missing methods” assist will not generate a nice clean async fn in this case, instead simply copying fn() -> impl Future<Output = ()> + Send.

Note that it doesn’t seem to apply to traits generated with trait-variant, but it’s still unfortunate to have to use procmacros to get a nice IDE experience.

Possibly related to #11315

Start with the “fill missing methods” assist described in the issue and compare its behavior with the related #11315 discussion. Done means implementations for trait methods returning impl Future<Output = ...> + Send are generated using async fn syntax where appropriate.