[analyzer][UninitializedObjectChecker] Reports Loc fields pointing to themselves

Description

[analyzer][UninitializedObjectChecker] Reports Loc fields pointing to themselves

I've added a new functionality, the checker is now able to
detect and report fields pointing to themselves. I figured
this would fit well into the checker as there's no reason
for a pointer to point to itself instead of being nullptr.

Differential Revision: https://reviews.llvm.org/D51305