[ICE] C-variadic functions are only valid with one or more fixed arguments

This ICEs (see title) because of the call:
```rust
extern {
    fn foo(...);
}

fn main() {
    unsafe { foo(1, 2, 3); }
}
```

That's because the definition of `foo` errored in 1.34, but doesn't since 1.35:
```rust
extern {
    fn foo(...);
}
```

cc @dlrobertson (found while working on https://github.com/rust-lang/rust/issues/44930#issuecomment-497163539)