[clang] [clang][analyzer] Split NullDereferenceChecker into a modeling and checker part (PR #122139)

[Balázs Kéri via cfe-commits]

balazske wrote:

Our plan is to add a new check for dereference of fixed address (like `(*0x111) = 1`). This is similar to the current `FixedAddressChecker` but has less false positives (if a fixed value is used as placeholder but never dereferenced). Additionally the existing checks for undefined value and label address dereferences could be moved into another checker (at least the checker name is not exact now).

https://github.com/llvm/llvm-project/pull/122139