Cleanup of `#[derive(Diagnostic)]` attribute parsers
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]](https://github.com/rust-lang/rust/pull/151657/changes/9e61014a8a0bb1f1d7911511c303a7ae2a9c2a7d)
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 https://github.com/rust-lang/rust/issues/151366 which cannot be parsed using `parse_args_with`.
- [Convert parse_nested_meta to parse_args_with for #[subdiagnostic]](https://github.com/rust-lang/rust/pull/151657/changes/5d21a21695d56b74ea249f269ee10195251008b7)
Same as above but for `#[subdiagnostic]`
- [Remove unused no_span option](https://github.com/rust-lang/rust/pull/151657/changes/0bf3f5d51cb853884240792818d81e70daec6ab7)
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](https://github.com/rust-lang/rust/pull/151657/changes/2e8347abf4147d2bffe4d7989a21b17ae04cdb57)
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)
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