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

[Mészáros Gergely via llvm-commits]

Maetveis wrote:

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

Thanks. Can I ask you to merge please, I don't have access.

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