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

[Hans Wennborg via cfe-commits]

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

lgtm

and +1 to what cor3ntin said about future direction.

In fact, it seems @Slartibarty already started this in #167200 but that patch didn't get noticed yet.

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