[clang] [Clang] [C++26] Implement P2573R2: `= delete("should have a reason");` (PR #86526)

[via cfe-commits]

https://github.com/cor3ntin approved this pull request.

Thanks for putting up with my comments and working on this feature.
I'm pretty happy with the state of the PR.

Please give it a day or two in case @erichkeane @AaronBallman @shafik @Fznamznon have further comments

https://github.com/llvm/llvm-project/pull/86526