Rollup merge of #100633 - estebank:must_use_async_fn_return, r=tmandry

Consider `#[must_use]` annotation on `async fn` as also affecting the `Future::Output`

No longer lint against `#[must_use] async fn foo()`.

When encountering a statement that awaits on a `Future`, check if the
`Future`'s parent item is annotated with `#[must_use]` and emit a lint
if so. This effectively makes `must_use` an annotation on the
`Future::Output` instead of only the `Future` itself.

Fix #78149.