C++20 renamed the `std::memory_order` enum's enumerators from `std::memory_order::memory_order_XYZ` to `std::memory_order::XYZ`. To maintain source compatibility with both C++17 and C++20, use `std::memory_order_XYZ`. https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0439r0.html https://en.cppreference.com/w/cpp/atomic/memory_order Differential Revision: https://phabricator.services.mozilla.com/D146395
