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

[estebank]

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.