[llvm] [gn] Add "/Zc:preprocessor" build flag on windows (PR #108252)

[via llvm-commits]

GkvJwa wrote:

> Maybe it shouldn't be passed when building with clang-cl? See [3f601a0](https://github.com/llvm/llvm-project/commit/3f601a0a8c2ddb9482c9eb54dbde8a7a074a7aad)

Fix, only for using msvc(cl.exe)

https://github.com/llvm/llvm-project/pull/108252