[clang-cl] Define _MSVC_LANG

Recently, Microsoft added support for a flag, /std, which controls which
version of the language rules MSVC should use.
MSVC hasn't updated __cplusplus though.
Instead, they added a new macro, _MSVC_LANG, which is defined in a
similar fashion to __cplusplus.  This is used to indicate which mode the
compiler is in.

llvm-svn: 273987