[PATCH] ignore -mconstructor-aliases when adding field paddings for asan

[Reid Kleckner]

No, even if the target dtor is implemented the derived class will have
extra padding it needs to unpoison. Consider:

struct A {
  virtual ~A();
  int x[1];
  int y[1];
};
struct B {
  virtual ~B();
  int z[1];
  int w[1];
};
B::~B() {}

The B dtor must unpoison the padding between z and w.

http://reviews.llvm.org/D6038