Add a diagram of how rustc types relate to one another

Often people have specific question of a thing they want to do: 'how do I get all function calls, as well as the DefId of the called function and the callee'? This isn't something the dev-guide can tell right away, because there's too many different things you can do with the compiler, but what we could do instead is show how to convert between types. Ideally this would be a graph-like structure, where every node connects to its neighbors, and it's easy to see how to get between them.

Start by surveying the rustc-dev-guide's existing compiler documentation and the rustc types referenced by common questions such as function calls and DefId. Define the graph's scope and relationships before documenting it. Done means the guide contains an easy-to-follow diagram showing how relevant rustc types convert to one another.