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

[via cfe-commits]

Sirraide wrote:

> Tests need to be fleshed out, I'd like to see template specializations + partial template specializations as well as special member functions and operators.

Forgot to mention it explicitly, but I’ve added more tests and implemented this for all the other sema diagnostics that had anything to do w/ explicitly deleted functions (there are a few that say something along the lines of ‘function has been implicitly deleted’, but I didn’t really look at those since this is about explicitly deleting functions).

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