rustdoc: Link "Trait Implementations" to sources

All methods listed in "Trait Implementations" now hyperlink to the source trait
instead of themselves, allowing easy browsing of the documentation of a trait
method.

Closes #17476

src/test/rustdoc/issue-17476.rs

@@ -0,0 +1,20 @@
+// 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.
+
+// aux-build:issue-17476.rs
+
+extern crate issue_17476;
+
+pub struct Foo;
+
+// @has issue_17476/struct.Foo.html \
+//      '//*[@href="http://example.com/issue_17476/trait.Foo.html#tymethod.foo"]' \
+//      'foo'
+impl issue_17476::Foo for Foo {}