Cleanup of #[derive(Diagnostic)] attribute parsers

[JonathanBrouwer]

This PR does a lot of refactoring on the implementation of #[derive(Diagnostic)]. It should have no observable effect other than error messages for incorrect usage of the attributes. In general, I think the error messages got better.

This PR can be reviewed commit by commit, each commit passes the tests.

• Convert parse_nested_meta to parse_args_with for #[diagnostic]
  Start parsing #[diagnostic] using syn's parse_args_with function instead of parse_nested_meta. This improves error messages and prepares for the new syntax needed for Tracking issue for MCP 959: Remove the fluent files #151366 which cannot be parsed using parse_args_with.
• Convert parse_nested_meta to parse_args_with for #[subdiagnostic]
  Same as above but for #[subdiagnostic]
• Remove unused no_span option
  Removes the no_span option of #[suggestion], which there were no tests for and which seems to have been unused. If needed again in the future, this can be re-added pretty easily, but I find that unlikely.
• Remove HasFieldMap trait in favour of passing FieldMap directly
  Removes the HasFieldMap trait, because I don't really see the point of having a trait "has a field map" if we can just pass the fieldmap itself instead.

r? @Kivooeo
(Thanks for reviewing my PRs so far :3)

[rustbot]

rustc_macros::diagnostics was changed

cc @davidtwco, @TaKO8Ki

[Kivooeo]

rustc_macros::diagnostics was changed

cc davidtwco, TaKO8Ki

hm, do we still need this notification?

In general, I think the error messages got better

yep, i think so too

Thanks for reviewing my PRs so far

i'll take a closer look tomorrow, but i'm pretty sure that's all fine <3

[JonathanBrouwer]

hm, do we still need this notification?

Afaik these notifications are for people who want to know if certain directories change, whether it is needed is mostly for them to decide. There is no further expectations such as waiting for their review. Actually just added myself to the list, cause I'm now also interested in changes of these files #151800

[Kivooeo]

Actually just added myself to the list

oh, that's nice, thanks for keeping an eye on it