[clang] [Clang][C++23] Implement P1774R8: Portable assumptions (PR #81014)

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Sirraide wrote:

> @Sirraide -- I'm not certain what email address to use to invite you to edit the document. Would you mind sending me an email ([aaron at aaronballman.com](mailto:aaron at aaronballman.com)) to let me know where you'd like the doc sent to?

Just the one my github account uses is fine, but I’ll send you an email anyway

https://github.com/llvm/llvm-project/pull/81014