Issue Description:

I ran into an issue with unowned vs weak that surprised me. The problem was clearly with me: due do things happening concurrently, I tried to dereference an unowned reference whose target was currently being deinitialized.

However, the debugger didn't help me figure out what the issue was: at the crash site, it would happily show the full object that I expected behind the reference. Ergo, I could not see at all why that line of code would fail.

Proposal: once deinit starts to run on any object instance, all unowned references to it should be shown to point to nil, just like for weak references, or (better yet) indicate that the value inaccessible because it's currently being deallocated/deinitialized.

(FWIW, I'm using the debugger from an IDE, so any warning should be output in a way that IDEs can pick up.