[clang] [clang][clang-cl] Add -fms-preprocessor-compat and support /Zc:preprocessor (PR #167200)

[Corentin Jabot via cfe-commits]

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@@ -307,12 +306,17 @@ Non-comprehensive list of changes in this release
 
 - Clang now rejects the invalid use of ``constexpr`` with ``auto`` and an explicit type in C. (#GH163090)
 
+- ``_MSVC_TRADITIONAL`` is now defined to ``0`` when using ``-fms-extensions``. The charize operator and ``/##/`` paste trick remain as supported extensions, which is a divergence from MSVC.
+
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cor3ntin wrote:

I don't think we want to diverge from MSVC in the other direction here, it just adds to the confusion.
@AaronBallman @erichkeane 

https://github.com/llvm/llvm-project/pull/167200