Migrate `codegen_ssa` to diagnostics structs - [Part 1] Initial migration of `codegen_ssa`. Going to split this crate migration in at least two PRs in order to avoid a huge PR and to quick off some questions around: 1. Translating messages from "external" crates. 2. Interfacing with OS messages. 3. Adding UI tests while migrating diagnostics. _See comments below._