Don't warn on fall-through from unreachable code.

Summary:
A motivating example:
class ClassWithDtor {
public:
  ~ClassWithDtor() {}
};
void fallthrough3(int n) {
  switch (n) {
    case 2:
      do {
        ClassWithDtor temp;
        return;
      } while (0);  // This generates a chain of unreachable CFG blocks.
    case 3:
      break;
  }
}

Reviewers: rsmith, doug.gregor, alexfh

Reviewed By: alexfh

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D330