[Analyzer] Iterator Checkers - Use the region of the topmost base class for iterators...

[Analyzer] Iterator Checkers - Use the region of the topmost base class for iterators stored in a region

If an iterator is represented by a derived C++ class but its comparison operator
is for its base the iterator checkers cannot recognize the iterators compared.
This results in false positives in very straightforward cases (range error when
dereferencing an iterator after disclosing that it is equal to the past-the-end
iterator).

To overcome this problem we always use the region of the topmost base class for
iterators stored in a region. A new method called getMostDerivedObjectRegion()
was added to the MemRegion class to get this region.

Differential Revision: https://reviews.llvm.org/D54466

llvm-svn: 348244