[analyzer] pr34404: Fix a crash on modeling pointers to indirect members.

We were crashing whenever a C++ pointer-to-member was taken, that was pointing
to a member of an anonymous structure field within a class, eg.

  struct A {
    struct {
     int x;
    };
  };
  // ...
  &A::x;

Differential Revision: https://reviews.llvm.org/D39800

llvm-svn: 319055