[SoM] Pointer to SwiftRuntimeLibrary.pltform.dll needs to be formalized

[stephen-hawley]

In ObjCProtocolCompiler.cs, we sometimes need to point been at the appropriate SwiftRuntimeLibrary assembly. Right now, the code roots through the file system and points at the one that's been built relative to the tom-swifty executable which is most assuredly the wrong thing to do and will likely fail in unit tests as well as in deployment.

The libraries will likely be installed somewhere formally and the code should instead look there.

Ideally, there should be a command line argument that can override this to make debugging easier, so really the code should pull the path out of the options.