[`unused_async`]: do not consider `await` in nested `async` blocks as used
Fixes#10800.
This PR makes sure that `await` expressions inside of inner `async` blocks don't prevent the lint from triggering.
For example
```rs
async fn foo() {
async {
std::future::ready(()).await;
}
}
```
Even though there *is* a `.await` expression in this function, it's contained in an async block, which means that the enclosing function doesn't need to be `async` too.
changelog: [`unused_async`]: do not consider `await` in nested `async` blocks as used
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