Solve the errors

Error 1 error C2681: 'add_rvalue_reference<_Tp*>::type' : invalid expression
type for dynamic_cast c:\libcxx\include\type_traits

This is one more step getting libcxx compile under Visual C++. The patch is
#if defined(_LIBCPP_MSVC) so will affect only this build.

When libcxx can be compiled, it will probably require the current version or
maybe even the next Update of Visual C++ 2013.

Patch by G M!