[Analyzer][NFC] Iterator Checkers - Separate iterator modeling and the actual checkers A monolithic checker class is hard to maintain. This patch splits it up into a modeling part, the three checkers and a debug checker. The common functions are moved into a library. Differential Revision: https://reviews.llvm.org/D70320 GitOrigin-RevId: afb13afcf2232c81fe8097832e5b6a2bde6bb3a5