Bug 43965 - Value of _MSVC_LANG doesn't match MSVC++ VS2019 /std:c++latest mode

Summary:
The patch adds correct value of _MSVC_LANG to match with MSVC++ VS2019 /std:c++latest mode.

    Bugzilla - Bug 43965
    https://bugs.llvm.org/show_bug.cgi?id=43965

    The value for a MS specific macro differs from Microsoft starting with VS2019 in /std:c++latest mode.

    -bash-4.2$ cat msvclang.cpp
    _MSVC_LANG

    -bash-4.2$ cl /std:c++latest -E msvclang.cpp
    Microsoft (R) C/C++ Optimizing Compiler Version 19.20.27508.1 for x64
    Copyright (C) Microsoft Corporation.  All rights reserved.

    /std:c++latest is provided as a preview of language features from the latest C++
    working draft, and we're eager to hear about bugs and suggestions for improvements.
    However, note that these features are provided as-is without support, and subject
    to changes or removal as the working draft evolves. See
    https://go.microsoft.com/fwlink/?linkid=2045807 for details.

    msvclang.cpp

    201705L

    -bash-4.2$ clang-cl /std:c++latest -E  msvclang.cpp
    201704L

Reviewers: rnk

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D70996