Fix MSVC build.

format-test.cc

@@ -292,9 +292,9 @@ std::string GetTestErrorMessage() {
       FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, 0,
       ERROR_FILE_EXISTS, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
       reinterpret_cast<LPWSTR>(&message), 0, 0);
-  ampl::UTF16ToUTF8 utf8_message(message);
+  fmt::internal::UTF16ToUTF8 utf8_message(message);
   LocalFree(message);
-  return std::string(utf8_message);
+  return fmt::str(utf8_message);
 }
 #endif