Mark all extern functions as nounwind

Unwinding across an FFI boundary is undefined behaviour, so we can mark
all external function as nounwind. The obvious exception are those
functions that actually perform the unwinding.

src/test/codegen/extern-functions.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![feature(unwind_attributes)]
+
+extern {
+// CHECK: Function Attrs: nounwind
+// CHECK-NEXT: declare void @extern_fn
+    fn extern_fn();
+// CHECK-NOT: Function Attrs: nounwind
+// CHECK: declare void @unwinding_extern_fn
+    #[unwind]
+    fn unwinding_extern_fn();
+}