[llvm] [llvm][cmake][Trivial] use /Zc:preprocessor with MSVC only explicitly (PR #111520)

[Markus Böck via llvm-commits]

https://github.com/zero9178 approved this pull request.

Makes sense to me given that the flag is effectively a workaround for MSVC internals and backwards compatibility 

https://github.com/llvm/llvm-project/pull/111520