rustdoc: Index inherent methods on primitives

The set of types which can have an inherent impl changed slightly and rustdoc
just needed to catch up to understand what it means to see a `impl str`!

Closes #23511

src/test/rustdoc/issue-23511.rs

@@ -0,0 +1,24 @@
+// 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.
+
+#![feature(no_std, lang_items, core)]
+#![no_std]
+
+extern crate core;
+
+pub mod str {
+    #![doc(primitive = "str")]
+
+    #[lang = "str"]
+    impl str {
+        // @has search-index.js foo
+        pub fn foo(&self) {}
+    }
+}