Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint This is more or less the same approach as https://github.com/rust-lang/rust/pull/152811, but in a much smaller size to make it reviewable. A lot of PRs will follow though. :) This PR creates the equivalent of `lint_level` working with `Diagnostic` and add new methods on `MultiSpan` to make it work as well (in particular because we need to copy messages/spans from one context to another). r? @JonathanBrouwer