[Project Layout] Refactor SwiftRuntime{,.Mac,.iOS}.csproj

[stephen-hawley]

There should be a common project referenced from each project.
Are these the only files in the ..\SwiftRuntimeLibary* folder and it's subfolders? If so consider using a globbing pattern to include them, or since this seems to be duplicated in the iOS project, use a shared project to add the files to both.

Originally posted by @mcumming in #807 (comment)