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

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cor3ntin wrote:

The last bullet point of your todo list should be done as a follow up PR. everything else can/should be done here.
I agree that back-porting this feature seems reasonable

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