[clang] Enable c++20 modules support for c++23preview in clang-cl (PR #173663)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

Other reviewers seem unavailable. Given the patch itself seems innocent, I'll land it.

https://github.com/llvm/llvm-project/pull/173663