[Analyzer] Iterator Checker - Part 2: Increment, decrement operators and ahead-of-begin checks

Add handling of the begin() funcion of containers to the iterator checkers,
together with the pre- and postfix ++ and -- operators of the iterators. This
makes possible the checking of iterators dereferenced ahead of the begin of the
container.

Differential Revision: https://reviews.llvm.org/D32642

llvm-svn: 335835