[clang] [clang] Define ATOMIC_FLAG_INIT correctly for C++. (PR #97534)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for catching this! Can you also add some tests to `clang/test/Headers/`? We have some coverage for stdatomic.h in C, but not in C++ (I'd recommend trying to add a `RUN` line to the .c file with `-x c++` so it's tested in C++ mode and then ensure there's sufficient coverage for `ATOMIC_FLAG_INIT`.)

https://github.com/llvm/llvm-project/pull/97534