Don’t declare async methods with alternatives in headers
If Swift sees this pattern of methods in an @objc protocol:
```
func hello() async -> Int
@available(*, renamed: “hello()”)
func hello(completion: @escaping (Int) -> Void)
```
Then PrintAsClang will print only the completion-handler-based method, not the async one, into a generated header, on the assumption that the completion-handler method may have greater availability.
Fixes rdar://94175167.
