unused_self: Don't trigger if the method body contains todo!()
If the author is using todo!(), presumably they intend to use self at some point later, so we don't have a good basis to recommend factoring out to an associated function.
Fixes#10117.
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changelog: Enhancement: [`unused_self`]: No longer lints, if the method body contains a `todo!()` call
[#10166](https://github.com/rust-lang/rust-clippy/pull/10166)
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