Auto merge of #114452 - weiznich:feature/diagnostic_on_unimplemented, r=compiler-errors

`#[diagnostic::on_unimplemented]` without filters

This commit adds support for a `#[diagnostic::on_unimplemented]` attribute with the following options:

* `message` to customize the primary error message
* `note` to add a customized note message to an error message
* `label` to customize the label part of the error message

The relevant behavior is specified in [RFC-3366](https://rust-lang.github.io/rfcs/3366-diagnostic-attribute-namespace.html)

Author: bors