[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273987 91177308-0d34-0410-b5e6-96231b3b80d8