[Async Refactoring] Wrap code in a continuation if conversion doesn't yield reasonable results

If we are requested to convert a function to async, but the call in the function’s body that eventually calls the completion handler doesn’t have an async alternative, we are currently copying the call as-is, replacing any calls to the completion handler by placeholders.

For example,
```swift
func testDispatch(completionHandler: @escaping (Int) -> Void) {
  DispatchQueue.global.async {
     completionHandler(longSyncFunc())
  }
}
```
becomes
```swift
func testDispatch() async -> Int  {
  DispatchQueue.global.async {
     <#completionHandler#>(longSyncFunc())
  }
}
```

and

```swift
func testUrlSession(completionHandler: @escaping (Data) -> Void) {
  let task = URLSession.shared.dataTask(with: request) { data, response, error in
    completion(data!)
  }
  task.resume()
}
```
becomes
```swift
func testUrlSession() async -> Data {
  let task = URLSession.shared.dataTask(with: request) { data, response, error in
    <#completion#>(data!)
  }
  task.resume()
}
```

Both of these are better modelled using continuations. Thus, if we find an expression that contains a call to the completion handler and can’t be hoisted to an await statement, we are wrapping the rest of the current scope in a `withChecked(Throwing)Continuation`, producing the following results:

```swift
func testDispatch() async -> Int {
  return await withCheckedContinuation { (continuation: CheckedContinuation<Int, Never>) in
    DispatchQueue.global.async {
      continuation.resume(returning: syncComputation())
    }
  }
}
```

and

```swift
func testDataTask() async -> Int?
  return await withCheckedContinuation { (continuation: CheckedContinuation<Data, Never>) in
    let task = URLSession.shared.dataTask { data, response, error in
      continuation.resume(returning: data!)
    }
    task.resume()
  }
}
```

I think both are much closer to what the developer is actually expecting.

Resolves rdar://79304583

Author: ahoppen