Refactor DereferenceChecker to use only the new Checker API instead of

Refactor DereferenceChecker to use only the new Checker API instead of
the old builder API.  This percolated a bunch of changes up to the
Checker class (where CheckLocation has been renamed VisitLocation) and
GRExprEngine.  ProgramPoint now has the notion of a "LocationCheck"
point (with PreLoad and PreStore respectively), and a bunch of the old
ProgramPoints that are no longer used have been removed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@86798 91177308-0d34-0410-b5e6-96231b3b80d8