[clang] [clang][X86] Add __cpuidex function to cpuid.h (PR #97785)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

Thanks - I think this is fine with me. In https://github.com/mingw-w64/mingw-w64/commit/a4c0c1d00d140d69a2c72f6ca0d49c91bdb2b08c we adjusted the mingw-w64 headers in anticipation that this gets merged in Clang 19, so it'd be good to have it settled before the 19.x branch gets made.

https://github.com/llvm/llvm-project/pull/97785