[clang] [clang-cl] Support the /std:c++23 option (PR #232659)

[Nico Weber via cfe-commits]

nico wrote:

> (ps: I agree not making the old spelling emit a warning, at least for now. And possibly for a long time.)

(Reasoning: It's a much better user experience if the previous release of the compiler supported the new thing suggested by the deprecation warning already, because then when you try to update the compiler and see the warning, you can flip to the new thing before you're done with the compiler update. This is often too expensive, but here it isn't.)

https://github.com/llvm/llvm-project/pull/232659