Fix parsing async keywords in void methods

[andrewchang-bird]

Overview

Methods returning Void implicitly aren’t parsed correctly for declaration keywords like throws and async which can lead to malformed code if the method body contains “async.”

func notAsync() {
  DispatchQueue.main.async {}
}

Test Plan

Added an additional test case to check for async false positives.

[jacobhzen]

This is great, possible to get it merged? We have several void async methods in our codebase