Generic return values cause an ICE in type_metadata

``` rust
fn foo<R>(r: R) -> R {
    r
}

pub fn main() {
    foo(0);
}
```

yields `error: internal compiler error: debuginfo: unexpected type in type_metadata`.

I presume this is a ty_infer, so I'm surprised we're seeing this in the debuginfo module while in the trans pass.
