[Pitch] Improve Interaction Between @main and Top Level Code

Hi all, This is a small pitch to address some sharp edges that get in the way of using @main in a single file executable, or one with a main.swift. The goal is to allow build systems to get rid of the inconsistent heuristics they currently use to build executables with -parse-as-library, and in general make parsing "just work" like users expect it to in more situations. The full proposal is here, including a more detailed discussion of some of the source compatibility implications and tradeoff...