Testing `Sync` diverges on recursive type

Typechecking the following program diverges:
```rust
#![feature(never_type)]

struct Foo<'a, T: 'a> {
    ph: std::marker::PhantomData<T>,
    foo: &'a Foo<'a, (T, T)>,
}

fn wub(f: Foo<!>) {
    sync(f)
}

fn sync<T: Sync>(x: T) {}

fn main() {}
```

Cc @eddyb @nikomatsakis 