deprecation of ATOMIC_FLAG_INIT.

Another option would have been to use macros to only initialize
std::atomic_flag before C++20, but I decided to use one compilation
path instead.

The major difference between std::atomic_flag and std::atomic<bool> is
that the former is guaranteed to be lock-free, but we already assume
std::atomic<bool> is lock-free in many places.

https://en.cppreference.com/w/cpp/atomic/atomic_flag

PiperOrigin-RevId: 487397075
Change-Id: I3f1c539ec8b2ca58547282e69ed73e93243e8efe