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

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llvmbot wrote:

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>> @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.
>
>That's good to know! I was going off the documentation at: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches because it says "issue" specifically. I'll post a patch to update those docs, thanks!

Error: Command failed due to missing milestone.

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