fix: make Debug impl for unions opaque

[zzau13]

Fixes #4118

[rustbot]

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[tgross35]

Thanks for working on this!

I'll take a more in-depth look soon, but why did this add core::cmp to Eq and PartialEq? If it is related to rustc-dep-of-std then that can be disregarded, we don't need to support both rustc-dep-of-std and extra-traits being enabled at the same time.

[rustbot]

Some changes occurred in solarish module

cc @jclulow, @pfmooney

Some changes occurred in OpenBSD module

cc @semarie

[tgross35]

Fyi the current error comes from a new lint in a recent nightly, I'll have it fixed soon #4177

[tgross35]

Feel free to cherry pick 99f4dd9 so you get CI results now, I can rebase once things turn green.

[tgross35]

Thank you for taking care of this!