Ignore not really redundant clones of ManuallyDrop

[tmiasko]

"Redundant" clones of ManuallyDrop are sometimes used for the side effect of
invoking the clone, without running the drop implementation of the inner type.
In other words, they aren't really redundant. For example, futures-rs crate:

#[allow(clippy::redundant_clone)] // The clone here isn't actually redundant.
unsafe fn increase_refcount<T: ArcWake>(data: *const ()) {
    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
    let arc = mem::ManuallyDrop::new(Arc::<T>::from_raw(data as *const T));
    // Now increase refcount, but don't drop new refcount either
    let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
}

changelog: Ignore redundant clone lint for ManuallyDrop.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Manishearth (or someone else) soon.

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[Manishearth]

@bors r+

Thanks!

[bors]

📌 Commit a5cdd4a has been approved by Manishearth

[bors]

⌛ Testing commit a5cdd4a with merge 61e3d8a...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 61e3d8a to master...