[analyzer] Initial commit for the upcoming refactoring of the IteratorChecker.

[analyzer] Initial commit for the upcoming refactoring of the IteratorChecker.

The new checker currently contains the very core infrastructure for tracking
the state of iterator-type objects in the analyzer: relating iterators to
their containers, tracking symbolic begin and end iterator values for
containers, and solving simple equality-type constraints over iterators.
A single specific check over this infrastructure is capable of finding usage of
out-of-range iterators in some simple cases.

Patch by Ádám Balogh!

Differential revision: https://reviews.llvm.org/D32592

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304160 91177308-0d34-0410-b5e6-96231b3b80d8