Expand #include_next in float.h from mingw to _msc_ver.

A test for this is checking if this compiles:

    #include <float.h>

    inline bool IsFinite(const double& number) {
     return _finite(number) != 0;
    }

That depends however on either mingw or msvc being installed, and
chapuni tells me there might be issues with float.h on mingw, so
no automated test is added.

llvm-svn: 155507