Allow expect on impl for derive_ord_xor_partial_ord

[profetia]

Closes #16298

This lint flags on the type defination, while also giving a warning on the span of the impl block. It is counter-intuitive that you need to put the expect on top of the type defination instead of the impl block to suppress the warning, and this PR addresses that.

changelog: [derive_ord_xor_partial_ord] allow expect on impl block

[rustbot]

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