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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM assuming precommit CI on Windows eventually comes back green.

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