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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I think this should be picked for the Clang 19 branch, so I filed https://github.com/llvm/llvm-project/issues/100362

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