Two new rustdoc tests for intra links

They both produce less-than-desirable output (links going to docs.rust-lang.org), but I haven't figured out yet how to assert about them properly.

src/test/rustdoc/intra-link-prim-methods-external-core.rs

@@ -0,0 +1,10 @@
+// aux-build:my-core.rs
+// ignore-cross-compile
+
+#![deny(intra_doc_link_resolution_failure)]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+//! A [`char`] and its [`char::len_utf8`].
+
+extern crate my_core;