[llvm] [NFC] [Support] Fix warning when build with clang-cl on Windows. (PR #65387)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The upstream code is https://github.com/openbsd/src/commit/b81763002452802e4f7304ea60f121253bd94bad　. It made another change, considering importing that change as well.

https://github.com/llvm/llvm-project/pull/65387