Auto merge of #117742 - weiznich:turn_overlapping_diagnostic_options_into_warnings, r=compiler-errors

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.

Author: bors