Auto merge of #6006 - ebroto:6001_unnecessary_sort_by, r=Manishearth

Restrict unnecessary_sort_by to non-reference, Copy types

`Vec::sort_by_key` closure parameter is `F: FnMut(&T) -> K`. The lint's suggestion destructures the `T` parameter; this was probably done to avoid different unnamed lifetimes when `K = Reverse<&T>`.

This change fixes two issues:
* Destructuring T when T is non-reference requires the type to be Copy, otherwise we would try to move from a shared reference. We make sure `T: Copy` holds.
* Make sure `T` is actually non-reference. I didn't go for destructuring multiple levels of references, as we would have to compensate in the closure body by removing derefs and maybe adding parens, which would add more complexity.

changelog: Restrict [`unnecessary_sort_by`] to non-reference, Copy types

Fixes #6001

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