Support extern type in rustdoc.

Fixes #45640.

src/test/rustdoc/foreigntype.rs

@@ -0,0 +1,28 @@
+// Copyright 2017 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.
+
+#![feature(extern_types)]
+
+extern {
+    // @has foreigntype/foreigntype.ExtType.html
+    pub type ExtType;
+}
+
+impl ExtType {
+    // @has - '//a[@class="fnname"]' 'do_something'
+    pub fn do_something(&self) {}
+}
+
+pub trait Trait {}
+
+// @has foreigntype/trait.Trait.html '//a[@class="foreigntype"]' 'ExtType'
+impl Trait for ExtType {}
+
+// @has foreigntype/index.html '//a[@class="foreigntype"]' 'ExtType'