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
