Rollup merge of #127591 - compiler-errors:label-after-primary, r=lcnr

Make sure that labels are defined after the primary span in diagnostics

Putting a `#[label]` before a `#[primary_span]` results in that label being overwritten, due to the semantics of `Diagnostic::span` and the fact that labels are stored in the `MultiSpan` of the diagnostic.

This isn't possible to fix in general, since a lot of code actually *relies* in this overwriting behavior (e.g. `rustc_on_unimplemented`). However, it's useful to enforce this for derive-diagnostics, since this is certainly never what you intend to do in a derived diagnostic, where all the fields are meaningful parts of the diagnostic being rendered.

This only matters for `#[label]`, since those are the ones stored in the `MultiSpan` of the error.

We could also make this "just work" by sorting the attrs or processing the primary span attr first, however I think it's kinda pointless to do.

There was 1 case where this mattered, but we literally didn't have a test exercising that diagnostic 🙃

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