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

[Louis Dionne via cfe-commits]

ldionne wrote:

@AaronBallman Just FYI you can also add the Milestone to the PR directly and use the /cherry-pick command right here. This is a bit simpler.

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