[LLVM] Check if LLVM_ON_UNIX is Defined
This is canonical in the rest of the repository and otherwise we can end
up with warnings when compiling with clang-cl on Windows that look like
the following:
```
2025-11-06T17:55:25.2412502Z C:\_work\llvm-project\llvm-project\llvm\include\llvm/Support/thread.h(37,5): warning: 'LLVM_ON_UNIX' is not defined, evaluates to 0 [-Wundef]
2025-11-06T17:55:25.2413436Z 37 | #if LLVM_ON_UNIX || _WIN32
2025-11-06T17:55:25.2413791Z | ^
2025-11-06T17:55:25.2414625Z C:\_work\llvm-project\llvm-project\llvm\include\llvm/Support/thread.h(52,5): warning: 'LLVM_ON_UNIX' is not defined, evaluates to 0 [-Wundef]
2025-11-06T17:55:25.2415585Z 52 | #if LLVM_ON_UNIX
2025-11-06T17:55:25.2415901Z | ^
2025-11-06T17:55:25.2416169Z 2 warnings generated.
```
Reviewers: joker-eph, pcc, cachemeifyoucan
Reviewed By: cachemeifyoucan
Pull Request: https://github.com/llvm/llvm-project/pull/166827
