message: test: don't use <experimental/filesystem> on macOS.
  Xcode 11 requires at least macOS 10.15 (upcoming) in order to use
  either <experimental/filesystem> or C++17 <filesystem>.
diff: #if defined(_LIBCPP_VERSION) && !defined(__APPLE__)
  #include <filesystem>
  #elif defined __has_include
  -#if __has_include(<experimental/filesystem>)
  +#if __has_include(<experimental/filesystem>) && !defined(__APPLE__)
  #include <experimental/filesystem>
  #endif
  #endif
  @@ -44,7 +44,7 @@ std::string makeTempDir(char* name_template) {
  name_template, strerror(errno)));
  #if defined(_LIBCPP_VERSION) && _LIBCPP_VERSION >= 9000 && !defined(__APPLE__)
  std::__fs::filesystem::create_directories(dirname);
  -#elif defined __cpp_lib_experimental_filesystem
  +#elif defined __cpp_lib_experimental_filesystem && !defined(__APPLE__)
  std::experimental::filesystem::create_directories(dirname);
  #endif
  #else
  @@ -102,7 +102,7 @@ void TestEnvironment::createParentPath(const std::string& path) {
  // We don't want to rely on mkdir etc. if we can avoid it, since it might not
  // exist in some environments such as ClusterFuzz.
  std::__fs::filesystem::create_directories(std::__fs::filesystem::path(path).parent_path());
  -#elif defined __cpp_lib_experimental_filesystem
  +#elif defined __cpp_lib_experimental_filesystem && !defined(__APPLE__)
  std::experimental::filesystem::create_directories(
  std::experimental::filesystem::path(path).parent_path());
  #else
  @@ -120,7 +120,7 @@ void TestEnvironment::removePath(const std::string& path) {
  return;
  }
  std::__fs::filesystem::remove_all(std::__fs::filesystem::path(path));
  -#elif defined __cpp_lib_experimental_filesystem
  +#elif defined __cpp_lib_experimental_filesystem && !defined(__APPLE__)
  if (!std::experimental::filesystem::exists(path)) {
  return;
  }
  