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

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Sirraide wrote:

> I think it's fine to refer to it as C++29 internally but I'd prefer to avoid user-facing mentions of C++29 until the committee ratifies the standard (this helps signal that using the language mode is subject to potentially breaking changes, etc).

We’ve already been supporting `-std=c++26` for a while though even when C++26 wasn’t though. Should we change that for C+29?

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