Don't transfer ownership to unowned references

[pkoronkevich]

Describe the bug When creating an Unowned reference to an Owned reference, the Owned reference loses ownership

To Reproduce

C@Owned ownedC = ... ;
C@Unowned unownedC = ownedC; // ownedC loses ownership

Expected behavior ownedC should retain ownership, while unownedC remains an Unowned reference.

[mcoblenz]

I think this is in preparation for annotation inference, in which case this will be the correct behavior. If we want to support this, we'll need a new operator.

[mcoblenz]

I think it's likely we need to add a new operator for this.