Missing links to src of implementors in docs of hir and ast visitors

In the docs for the compiler no links to the source code of the implementors of the [ast::Visitor](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/trait.Visitor.html#implementors) and the [hir::Visitor](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/intravisit/trait.Visitor.html#implementors) are provided. @jyn514 said that this is likely a bug.