We're using a small custom frontend tool to generate indexstore data for `.swiftinterface` files in the SDKs. We do this by treating the `.swiftinterface` file as the input of an interface compilation, but this exits early because it treats it as a `SourceFile` instead of an external `LoadedFile`. This happens even if we call `setIsSystemModule(true)` unless we skip setting the SDK path, but that causes other problems. It seems harmless to check for `SourceFile`s as well, so that a tool processing an SDK interface as a direct input still gets the right state.
