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

[Erich Keane via cfe-commits]

erichkeane wrote:

> Wait for CI green, but seems windows bots fall into a trouble.

Windows bot is typically pretty far behind(usually 4-5 hrs during the day), but 13 hours is a heck of a delay!  I think all we can do is wait :/

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