[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:

Thanks for all the reviewers! The last CI was green, but there was a conflict in ReleaseNotes. After I merged the main branch and resolved the conflict, there was a build issue with CI, but it was not caused by this PR(Seems the root cause is flang). therefor, I'll merge this PR.

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