[unused_async]: do not consider await in nested async blocks as used

[y21]

Fixes #10800.
This PR makes sure that await expressions inside of inner async blocks don't prevent the lint from triggering.
For example

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

[rustbot]

r? @Jarcho

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[Jarcho]

Thank you. @bors r+

[bors]

📌 Commit 8ef6240 has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8ef6240 with merge 05740ad...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing 05740ad to master...