[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
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