The compiler is the Clang front end over their MSVC backend, http://blogs.msdn.com/b/vcblog/archive/2015/05/01/bringing-c... . I guess I wouldn't be too surprised if it was open sourced later as that seems to be the direction they are generally moving in.
http://blogs.windows.com/buildingapps/2015/08/06/windows-bri...
"(we’re not open sourcing the compiler)"