[clang-cl] Implement support for /std

/std: supports two arguments, c++14 and c++latest.  Currently, c++latest
maps to c++1z but this might change down the road.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273841 91177308-0d34-0410-b5e6-96231b3b80d8