[llvm] [LLVM] Check if LLVM_ON_UNIX is Defined (PR #166827)

[Aiden Grossman via llvm-commits]

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boomanaiden154 wrote:

No. If it's set to false inside CMake, it gets undefined rather than set to zero.

https://cmake.org/cmake/help/latest/command/configure_file.html#transformations

https://github.com/llvm/llvm-project/pull/166827