ICE: Autoderef but type not derefable on fmt!("%?", None)

This is probably bad code because there's no way for the compiler to guess the complete type of `None`, but maybe it could be an explicit failure.

```
foo.rs:1:17: 1:21 error: internal compiler error: Autoderef but type not derefable: [type error]
foo.rs:1 fn main() { fmt!("%?", None); }
                          ^~~~
note: in expansion of fmt!
foo.rs:1:12: 1:29 note: expansion site
```
