`extern_in_paths` feature fails when `extern` starts a type

The following test:

```rust
#![feature(extern_in_paths)]

fn foo<T>() { }

fn main() {
    foo::<extern::std::fmt::Formatter>();
}
```

Gives me:

```
error: expected `fn`, found `::`
 --> src/main.rs:6:17
  |
6 |     foo::<extern::std::fmt::Formatter>();
  |                 ^^ expected `fn` here
```

cc @petrochenkov 