[clang] [Clang] Add flags and lang option for C++2d (PR #203992)

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Sirraide wrote:

> You should also change
> 
> https://github.com/llvm/llvm-project/blob/80ad13af63ef25d4e62adce6e3362f659b5c521f/clang/www/make_cxx_dr_status#L186-L191
> 
> Ideally I'd also want a new RUN line in every C++ DR test, but I can handle that myself afterwards.

Done

https://github.com/llvm/llvm-project/pull/203992