2.9 was the first version of Clang with C++11 stuff (outside of the features which everyone happened to support as extensions to C++03), released several months before C++/CX in April 2011. It's easy to forget these days with how impressive their progress has been, but it wasn't that long ago that Clang was the neat project that people hoped would turn into a usable compiler. The C++14 bits used weren't even proposed yet when C++/CX was released, so C++/CX may have been needed even if VC++ was fully caught up with the standards.

C++/CX came out in 2011. The C++14 constexpr stuff didn't make it into Clang and gcc until last year.