[clang] [Clang] Define `__SIG_ATOMIC_TYPE__` macro (PR #213934)

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llvmbot wrote:

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Failed to cherry-pick: c08ed4c1ab086aad6f22cfc499e74d92953045ae

https://github.com/llvm/llvm-project/actions/runs/30978960770

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/213934