`IntoDiagnostic` defaults to `ErrorGuaranteed`, because errors are the most common diagnostic level. It makes sense to do likewise for the closely-related (and much more widely used) `DiagnosticBuilder` type, letting us write `DiagnosticBuilder<'a, ErrorGuaranteed>` as just `DiagnosticBuilder<'a>`. This cuts over 200 lines of code due to many multi-line things becoming single line things.