Comment # 7 on bug 179 from Iain Buclaw

There seems to be a key thing happening here:

---
struct SignalImpl
{
    @disable this(this);  // SignalImpl is now non-POD
}
---

Because of this, TypeFunction::toCtype sets TREE_ADDRESSABLE on functions
returning SignalImpl (which is correct behaviour, we do *not* want non-POD
types to return in registers).

However...
---
ref RestrictedSignal valueChanged()
{
    return valueChangedSg.mRestricted;  // returned by ref
}
---

Reference returns should not have TREE_ADDRESSABLE set as they are already
returning in memory.