It ships clang, because that is Microsoft's answer to C support on Windows, for those that still want to use C.
Visual C++ focus is naturally C++.
C with extensions, by definition, is not C, rather something else.
Swift interoperates with Objective-C, thus C interoperability comes for free.
IO Kit, DriverKit, Metal Shaders, LLVM make all use of C++.
Objective-C is C, with extensions. Swift interoperates with C, not C++.