[analyzer] Improve CastToStruct checker so it can also detect widening casts of struct data

Example:

struct AB {
  int A;
  int B;
};

struct ABC {
  int A;
  int B;
  int C;
};

void f() {
  struct AB Data;
  struct ABC *P = (struct ABC *)&Data;
}

Differential Revision: https://reviews.llvm.org/D23508

llvm-svn: 282411