[clang] Fix clang reject valid C++ code after d999ce0302f06d250f6d496b56a5a5f (PR #94471)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

Oh, we need to adjust https://github.com/root-project/root/blob/be5d34934de883270683030b3af2cd1195d17ea8/cmake/modules/RootMacros.cmake#L272 to skip in case of C++...

https://github.com/llvm/llvm-project/pull/94471