Support extern opaque types.

```rust
#![feature(extern_types)]

extern "C" {
    type A;
    fn foo() -> Box<A>;
    fn bar();
}

impl A {
    fn bar(&self) {
        unsafe {bar()};
    }
}

fn main() {
    let a = unsafe { foo() } ;
    a.bar();
}
```
This code compiles fine in rustc, but rust-analyzer gives `Box<{unknown}>` inlay hint to a.