Don't drop blocks on foreign functions

A code like

```rust
extern "C" {
    fn f() {
        fn g() {}
    }
}
```

is incorrect and does not compile. Today rustfmt formats this in a way
that is correct:

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

But this loses information, and doesn't have to be done because we know
the content of the block if it is present. During development I don't
think rustfmt should drop the block in this context.

Closes #4313

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