Fix handling of ignored arguments in FFI wrappers for rust functions

Fixes #16441.

src/test/run-pass/issue-16441.rs

@@ -0,0 +1,17 @@
+// 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.
+
+struct Empty;
+
+// This used to cause an ICE
+extern "C" fn ice(_a: Empty) {}
+
+fn main() {
+}