[clang-cl] Define _MSVC_LANG for -std=c++2b

This matches the value that msvc v19.29 VS16.11 uses for
_MSVC_LANG with /std:c++latest.

Differential Revision: https://reviews.llvm.org/D114952

GitOrigin-RevId: 2a2b3a3e3df7b89e18be9ffab1e08d7ca578cf57
2 files changed