[clang] [Clang][C++23] Core language changes from P1467R9 extended floating-point types and standard names. (PR #78503)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

What's the status of this PR?  Seems like it would be helpful for some of the C23 additions (`_Float128` being of most interest to me)

https://github.com/llvm/llvm-project/pull/78503