Use a visitor to look for non-FFI-safe types

Fixes #16250.

src/test/compile-fail/issue-16250.rs

@@ -0,0 +1,18 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(warnings)]
+
+extern {
+    pub fn foo(x: (int)); //~ ERROR found rust type `int` in foreign module
+}
+
+fn main() {
+}