[analyzer] Check for returning null references in ReturnUndefChecker.

[analyzer] Check for returning null references in ReturnUndefChecker.

Officially in the C++ standard, a null reference cannot exist. However,
it's still very easy to create one:

int &getNullRef() {
  int *p = 0;
  return *p;
}

We already check that binds to reference regions don't create null references.
This patch checks that we don't create null references by returning, either.

<rdar://problem/13364378>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176601 91177308-0d34-0410-b5e6-96231b3b80d8