Summary: @public Infer previously did not work correctly when a function returns the result of a skip function:
```
retUndef() {
x = undefined();
return x;
}
derefUndef() {
y = retUndef();
y.doSomething(); // Symexec_memory_error here, prevents spec inference
}
```
The problem is that angelic mode did not know to add the return value of `retUndef()` to the footprint.
This diff fixes the problem by adding return values marked with the `Aundef` attribute to the footprint.
This is done lazily (e.g., a value only gets added to the footprint when you try to deref it).
Reviewed By: @jvillard
Differential Revision: D2444929
Sam Blackshear