message: [debugserver] Switch back to std::once_flag
  We cannot use llvm::once_flag in debugserver because doesn't link
  against llvm.
diff: @@ -46,7 +46,7 @@ static os_activity_stream_set_event_handler_t
  s_os_activity_stream_set_event_handler;
  bool LookupSPICalls() {
  - static llvm::once_flag s_once_flag;
  + static std::once_flag s_once_flag;
  static bool s_has_spi;
  std::call_once(s_once_flag, [] {
  