This solves warning C4005: '_NOEXCEPT' : macro redefinition when compiling
with Visual C++ 2013 by making libcxx definition text-identical to yvals.h.

Persumably this definition is for older Visual C++ versions.
In such cases it will still be defined so no functionality change.

Other platforms should not be affected as this is inside
#elif defined(_LIBCPP_MSVC)

Patch by G M!