Add some additional warnings for duplicated diagnostic items This commit adds warnings if a user supplies several diagnostic options where we can only apply one of them. We explicitly warn about ignored options here. In addition a small test for these warnings is added. r? `@compiler-errors` For now that's the last PR to improve the warnings generated by misused `#[diagnostic::on_unimplemented]` attributes. I'm not sure what needs to be done next to move this closer to stabilization.