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

[Donát Nagy via cfe-commits]

NagyDonat wrote:

I converted this PR to draft because I'll examine the ideas described in the inline comment https://github.com/llvm/llvm-project/pull/132250#discussion_r2007710175 . I'll probably tweak the framework based on those ideas, then revisit refactoring this checker.

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