message: Fix windows unit tests
  The unit test I added in the previous commit discovered a bug in
  PrependPathComponent on windows -- it was calling SetFile with the host native
  path syntax, whereas it should be explicitly specifying the path syntax (as
  AppendPathComponent does). This fixes it.
diff: @@ -1277,7 +1277,7 @@ void FileSpec::PrependPathComponent(llvm::StringRef component) {
  std::string result =
  join_path_components(m_syntax, {component, m_directory.GetStringRef(),
  m_filename.GetStringRef()});
  - SetFile(result, resolve);
  + SetFile(result, resolve, m_syntax);
  }
  void FileSpec::PrependPathComponent(const FileSpec &new_path) {
  