[clang] [-Wunsafe-buffer-usage] Add FixableGadget for AddAssign in UnspecifiedUntypedContext (PR #71862)

[Artem Dergachev via cfe-commits]

https://github.com/haoNoQ commented:

Aha, yes, this is exactly what I had in mind with the fix-around-it thing!

I found one more nitpick, but I think everything else looks great!

https://github.com/llvm/llvm-project/pull/71862