[clang] [clang-tools-extra] [clang-tidy][dataflow] Add `bugprone-null-check-after-dereference` check (PR #84166)

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martinboehme wrote:

> gentle ping @martinboehme @gribozavr

Sorry, I didn't notice your original comment that this is ready for review again.

I've added a few individual comments, but I haven't reviewed the whole PR in depth because I see that there are still quite a number of old comments that haven't been addressed. Please could you address these comments before the next review round?

Also:

> I'm still yet to refactor handling `Top`, currently it's used in early returns where possible, but I still need to think of a clear way to make central functions for it.

Can you address this too before the next review round? It's hard to review a PR that the author themself acknowledges isn't in a final state yet.

If you're not sure what the best way is to do certain things, happy to help with that of course -- but then we should address those specific topics first before continuing to review the whole PR.

https://github.com/llvm/llvm-project/pull/84166