[analyzer] BugReporter - more precise tracking of C++ references

When BugReporter tracks C++ references involved in a null pointer violation, we
want to differentiate between a null reference and a reference to a null pointer. In the
first case, we want to track the region for the reference location; in the second, we want
to track the null pointer.

In addition, the core creates CXXTempObjectRegion to represent the location of the
C++ reference, so teach FindLastStoreBRVisitor about it.

This helps null pointer suppression to kick in.

(Patch by Anna and Jordan.)

llvm-svn: 176969