Add compile-fail test for unwind_attributes

Test copied from src/test/codegen/extern-functions.rs.

Refs: rust-lang#39059

src/test/compile-fail/feature-gate-unwind-attributes.rs

@@ -0,0 +1,28 @@
+// 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
+
+#![crate_type = "lib"]
+
+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] //~ ERROR #[unwind] is experimental
+    fn unwinding_extern_fn();
+}
+
+pub unsafe fn force_declare() {
+    extern_fn();
+    unwinding_extern_fn();
+}

src/tools/tidy/src/features.rs

@@ -167,7 +167,7 @@ pub fn check(path: &Path, bad: &mut bool) {
 
     // FIXME get this whitelist empty.
     let whitelist = vec![
-        "cfg_target_thread_local", "unwind_attributes",
+        "cfg_target_thread_local",
     ];
 
     // Only check the number of lang features.