swift

da0a3311 - Add `-async-main` flag to favor asynchronous main

Add `-async-main` flag to favor asynchronous main This flag biases the overload checker in favor of selecting an asynchronous main function over a synchronous main. If no asynchronous main function exists, a synchronous one will still be selected. Likewise, if the flag is not passed and there are only asynchronous main functions available, the most specific asynchronous main function will still be selected.