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

[via cfe-commits]

https://github.com/cor3ntin commented:

Last batch of nitpicks, and then I think I'll be happy enough to approve this patch!

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