[clang-cl] Set _MSVC_LANG to 202002L with /std:c++20

MSVC says this should be 202002L for /std:c++20, and of VS16.11
that's indeed the case (older versions warn that they don't
understand /std:c++20, and then cl.exe defaults to C++14 and
sets _MSVC_LANG to 201402 accordingly).

Differential Revision: https://reviews.llvm.org/D114867