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

[Aiden Grossman via llvm-commits]

================

----------------
boomanaiden154 wrote:

Yeah. Every other place I could find that checks `LLVM_ON_UNIX` checks if it is defined rather than if it is zero, so definitely a bit of work to get things switched, and I'm not sure it's worth the effort.

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