`SwiftASTContext` does not properly locate the runtime libraries on Windows

[compnerd]

The SwiftASTContext does not properly identify the library path and relies on the process loading process to resolve the path to the requisite DLLs and map them in. This is most apparent in the REPL path where an extra -L%ProgramFiles%\swift\runtime-development\usr\bin is required (which is very confusing given that Windows links against the import libraries and not the DLLs). This needs to be resolved so that we can properly evaluate expressions and set breakpoints.