ruff_db: add primary annotation message mutators on `Diagnostic`
This will enable us to provide an API on `LintDiagnosticGuard` for
setting the primary annotation message. It will require an `unwrap()`,
but due to how `LintDiagnosticGuard` will build a `Diagnostic`, this
`unwrap()` will be guaranteed to succeed. (And it won't bubble out to
every user of `LintDiagnosticGuard`.)
