Rollup merge of #85200 - FabianWolff:issue-84647, r=nikomatsakis

Ignore derived Clone and Debug implementations during dead code analysis

This pull request fixes #84647. Derived implementations of `Clone` and `Debug` always trivially read all fields, so "field is never read" dead code warnings are never triggered. Arguably, though, a user most likely will only be interested in whether _their_ code ever reads those fields, which is the behavior I have implemented here.

Note that implementations of `Clone` and `Debug` are only ignored if they are `#[derive(...)]`d; a custom `impl Clone/Debug for ...` will still be analyzed normally (i.e. if a custom `Clone` implementation uses all fields of the struct, this will continue to suppress dead code warnings about unused fields); this seemed like the least intrusive change to me (although it would be easy to change — just drop the `&& [impl_]item.span.in_derive_expansion()` in the if conditions).

The only thing that I am slightly unsure about is that in #84647, `@matklad` said
> Doesn't seem easy to fix though :(

However, it _was_ pretty straightforward to fix, so did I perhaps overlook something obvious? `@matklad,` could you weigh in on this?

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