[analyzer] If a lazy binding is undefined, pretend that it's unknown instead.

This is a hack to work around the fact that we don't track extents for our
default bindings:

  CGPoint p;
  p.x = 0.0;
  p.y = 0.0;
  rectParam.origin = p;
  use(rectParam.size); // warning: uninitialized value in rectParam.size.width

In this case, the default binding for 'p' gets copied into 'rectParam',
because the 'origin' field is at offset 0 within CGRect. From then on,
rectParam's old default binding (in this case a symbol) is lost.

This patch silences the warning by pretending that lazy bindings are never
made from uninitialized memory, but not only is that not true, the original
default binding is still getting overwritten (see FIXME test cases).
The long-term solution is tracked in <rdar://problem/12701038>

PR14765 and <rdar://problem/12875012>