Add a custom `Res` type
Add a custom `Res` type

- Don't make rustc_resolve::Res public; use a new type alias instead

graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust