[clang] [clang] Predefine `_MSVC_TRADITIONAL` in MSVC compatibility mode (PR #184278)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

This makes sense. 

As future work here, I think we should support `/Zc:preprocessor` that would disable msvc compatible preprocessor behavior, as more and more windows user use the new, more conforming, msvc preprocessor.

https://github.com/llvm/llvm-project/pull/184278