feat: implement attribute completions for diagnostics module

[rmehri01]

Implements completions for diagnostic::on_unimplemented and diagnostic::do_not_recommend. Unless I missed it, I don't think there are other attributes here that deal with qualifiers like diagnostic:: so I added somewhat naive splitting on :: and then made it so that it is stripped before being used in the label and snippet. Similarly, in complete_known_attribute_input we first need to check if the qualifier is present instead of assuming that as_single_name_ref will work.

I also noticed that must_use is currently duplicated in functions and traits since it already shows up in item 😄

Closes #19892

[Veykril]

Thanks! I'll do a small follow up change to this