[clang] [clang-cl]: Add /std:c++23preview and update _MSVC_LANG for C++23 (PR #112378)

[via cfe-commits]

zmodem wrote:

> LGTM! Can you also add a release note to clang/docs/ReleaseNotes.rst so users know about the new flag?

Sure, will do.

https://github.com/llvm/llvm-project/pull/112378