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

[Aaron Ballman via cfe-commits]

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

In general, this LGTM, thank you! I did spot some minor stuff, but feel free to fix it up when landing.

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