[clang] [clang] Reject if constexpr in C (PR #112685)

[via cfe-commits]

llvmbot wrote:

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Failed to cherry-pick: e21c80ac73a9da5c86c20dbce37c9227a17ab06a

https://github.com/llvm/llvm-project/actions/runs/11384260535

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/112685