[analyzer] ObjCContainers: Track index values.

Use trackExpressionValue() (previously known as trackNullOrUndefValue())
to track index value in the report, so that the user knew
what Static Analyzer thinks the index is.

Additionally, implement printState() to help debugging the checker later.

Differential Revision: https://reviews.llvm.org/D55458

llvm-svn: 349227