rustdoc: Don't duplicate inlined impl blocks

Closes #21474

src/test/rustdoc/issue-21474.rs

@@ -0,0 +1,21 @@
+// Copyright 2012-2014 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.
+
+pub use inner::*;
+
+mod inner {
+    impl super::Blah for super::What { }
+}
+
+pub trait Blah { }
+
+// @count issue_21474/struct.What.html \
+//        '//*[@class="impl"]' 1
+pub struct What;