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

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Sirraide wrote:

So, the message is now stored in the `DefaultFunctionInfo`, which I’ve renamed to `ExtraFunctionInfo`; because setting the deleted message may require allocating a `DefaultFunctionInfo`, I’ve added a separate function for that (`FunctionDecl::setDeletedMessage()`).

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