Rollup merge of #152933 - GuillaumeGomez:start-migrating-lintdiag, r=JonathanBrouwer

Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint

This is more or less the same approach as rust-lang/rust#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

Author: JonathanBrouwer