message: [Host] Use FileSystem wrapper
  Fixes Host.mm to use the FileSystem class instead of making native calls
  to check if a file exists.
diff: @@ -1273,21 +1273,19 @@ static bool ShouldLaunchUsingXPC(ProcessLaunchInfo &launch_info) {
  Status Host::LaunchProcess(ProcessLaunchInfo &launch_info) {
  Status error;
  +
  + FileSystem &fs = FileSystem::Instance();
  FileSpec exe_spec(launch_info.GetExecutableFile());
  - llvm::sys::fs::file_status stats;
  - status(exe_spec.GetPath(), stats);
  - if (!exists(stats)) {
  + if (!fs.Exists(exe_spec))
  FileSystem::Instance().Resolve(exe_spec);
  - status(exe_spec.GetPath(), stats);
  - }
  - if (!exists(stats)) {
  +
  + if (!fs.Exists(exe_spec))
  FileSystem::Instance().ResolveExecutableLocation(exe_spec);
  - status(exe_spec.GetPath(), stats);
  - }
  - if (!exists(stats)) {
  +
  + if (!fs.Exists(exe_spec)) {
  error.SetErrorStringWithFormatv("executable doesn't exist: '{0}'",
  - launch_info.GetExecutableFile());
  + exe_spec);
  return error;
  }
  