Properly translate methods with foreign CC

This fixes a general issue of trying to define extern functions inside impl blocks resulting in
ICE.

Fixes #21238
Fixes #20734
Fixes #19047

src/test/run-pass/extern-methods.rs

@@ -0,0 +1,35 @@
+// Copyright 2015 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.
+trait A {
+    extern "fastcall" fn test1(i: i32);
+    extern fn test2(i: i32);
+}
+
+struct S;
+impl S {
+    extern "stdcall" fn test3(i: i32) {
+        assert_eq!(i, 3);
+    }
+}
+
+impl A for S {
+    extern "fastcall" fn test1(i: i32) {
+        assert_eq!(i, 1);
+    }
+    extern fn test2(i: i32) {
+        assert_eq!(i, 2);
+    }
+}
+
+fn main() {
+    <S as A>::test1(1);
+    <S as A>::test2(2);
+    S::test3(3);
+}