Rollup merge of #102612 - JhonnyBillM:migrate-codegen-ssa-to-diagnostics-structs, r=davidtwco

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._

Author: matthiaskrgr