[clang] [NFC][analyzer] Multipart checker refactor 2: NullabilityChecker (PR #132250)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

I introduced the new "checker family" framework for the multipart checkers, so now this PR is superseded by https://github.com/llvm/llvm-project/pull/143735 which converts NullabilityChecker to _that_ framework.

https://github.com/llvm/llvm-project/pull/132250