Implement -Z hir-stats for nested foreign items

Implement -Z hir-stats for nested foreign items

An attempt to compute HIR stats for crates with nested foreign items results in an ICE.

```
fn main() {
    extern "C" { fn f(); }
}
```

```
thread 'rustc' panicked at 'visit_nested_xxx must be manually implemented in this visitor'
```

Provide required implementation of visitor method.