Handle `rustc_on_unimplemented` in duplicated_attributes

[Noratrieb]

#[rustc_on_unimplemented(
    on(
        _Self = "&str",
        label = "`a"
    ),
    on(
        _Self = "alloc::string::String",
        label = "a"
    ),
)]

The lint treats this as a repetition because rustc_on_unimplemented::on::label appears twice, but that's ok.

Fixes #12619

changelog: [duplicated_attributes]: fix handling of rustc_on_unimplemented

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[Noratrieb]

Oops, that's what I get for throwing something up quickly before going to bed. Will fix (or someone else can fix it for me if you want to :D).

[bors]

:umbrella: The latest upstream changes (presumably #12646) made this pull request unmergeable. Please resolve the merge conflicts.

[Noratrieb]

r? xFrednet