message: constexpr std::string_view -> const std::string_view
  This works around gcc 7.2/clang rC347417 which have a bad interaction with libstdc++'s implementation of P0426
  constexpr std::string_view also emits a string_view object in .rodata that cannot be optimized out by clang (which means larger object file size)
  So use good old const.
diff: @@ -412,8 +412,8 @@ void LaunchStdin() {
  std::thread([]() {
  set_thread_name("stdin");
  std::string str;
  + const std::string_view kContentLength("Content-Length: ");
  while (true) {
  - constexpr std::string_view kContentLength("Content-Length: ");
  int len = 0;
  str.clear();
  while (true) {
  