[analyzer] Fix a crash on casting symbolic pointers to derived classes.

Commit r340984 causes a crash when a pointer to a completely unrelated type
UnrelatedT (eg., opaque struct pattern) is being casted from base class BaseT to
derived class DerivedT, which results in an ill-formed region
Derived{SymRegion{$<UnrelatedT x>}, DerivedT}.

Differential Revision: https://reviews.llvm.org/D52189

llvm-svn: 343051