Usage of std::atomic_flag::test() added in C++20 compiles successfully with --std=c++17

[magreenblatt]

Usage of the std::atomic_flag::test() method added in C++20 should fail to compile with --std=c++17. Instead, it compiles successfully,

I'm not an expert here, but I think the code needs to check the _LIBCPP_STD_VER value.

CC @ogiroux who added this functionality in https://reviews.llvm.org/D68480

[JoeLoser]

This is also a problem for the member functions wait(), notify_one(), and notify_all within std::atomic_flag class it looks like. All of them should be only available for C++20 or later.