[clang] [Clang] Allow the value of unroll count to be zero in `#pragma GCC unroll` and `#pragma unroll` (PR #88666)

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yronglin wrote:

Sorry, I didn't find this problem last time I reviewed https://github.com/llvm/llvm-project/pull/89567. @alexfh Seems this issue introduced by https://github.com/llvm/llvm-project/pull/89567, I've a PR to fix it https://github.com/llvm/llvm-project/pull/90240. 

https://github.com/llvm/llvm-project/pull/88666