`async_without_await` triggers on `@concurrent` functions without `await`

The rule is supposed to flag `async` functions which don't contain any `await` expression/statement. However, `@concurrent` functions in Swift 6.2 require the `async` in any case. The rule needs to exclude `@concurrent` functions.

Example:

```swift
@concurrent
func f() async {} // rule triggers here even though it must not
```