Add edge labels to our graphviz output
Add edge labels to our graphviz output

This adds the `EdgeKind` as a label to edges in the graphviz IR dump
output. This is useful for understanding why something is referenced by another
thing.

emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen