[analyzer] Ensure BugReporterTracking works on regions with pointer arithmetic

Introduce a new helper function, which computes the first symbolic region in
the base region chain. The corresponding symbol has been used for assuming that
a pointer is null. Now, it will also be used for checking if it is null.

This ensures that we are tracking a null pointer correctly in the BugReporter.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179916 91177308-0d34-0410-b5e6-96231b3b80d8