Visual Studio 2015 Update 3 introduced the notion of language standard
levels to MSVC.  The language standard level is defined in `_MSVC_LANG`
instead of `__cplusplus`.  It also added support for the `-std:c++14`
and `-std:c++latest` flags, although the compiler defaults to its C++14
mode anyway.  Visual Studio 2017 Update 3 will introduce support for the
`-std:c++17` flag.

Fixes: #16482