Follow through references to catch returned stack addresses, local blocks,...

Follow through references to catch returned stack addresses, local blocks, label addresses or references to temporaries, e.g:

const int& g2() {
  int s1;
  int &s2 = s1; // expected-note {{binding reference variable 's2' here}}
  return s2; // expected-warning {{reference to stack memory associated with local variable 's1' returned}}
}

llvm-svn: 120483