There appear to be two independent issues:

1. os.h:386:16: error: declaration of 'buffer_size' with internal linkage cannot be exported. constexpr implying internal linkage has an exception for module interface units, but it seems when enclosed within extern "C++" clang ceases to apply this exception. I suspect clang is probably wrong here and fmt/MSVC are correct, but not 100% sure. I filed an issue with LLVM.
2. error: declaration of 'assert_fail' in module fmt follows declaration in the global module. This one I suspect is more likely to be clang being correct and fmt/MSVC wrong to permit it, but again unsure. clang seems to want the definitions of things previously declared within extern "C++" to also be within such a block.

So it's possible there is no error here on fmt's side, but from experience, with two discrepancies between MSVC and clang, MSVC being on the correct side of both is unlikely!