Per-byte handling for UnsafePinned

[LaneAsade]

Fixes #4819

This PR ensures that UnsafePinned is handled in per-byte way just like UnsafeCell by adding a check for UnsafePinned alongside the existing UnsafeCell check in visit_value. This makes sure that only the bytes actually covered by the UnsafePinned opt out and not the entire allocation.

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[RalfJung]

Thanks! However, I think it is too early for this. We currently still honor the Unpin trait, for which it is rather unclear what it even means to do things per-byte. We have to first land rust-lang/rust#152946 before we can remove the Unpin hack from Miri, and then we can consider doing per-byte UnsafePinned.

Also, this PR treats UnsafeCell and UnsafePinned the same, which I don't think is right.
And finally, the test doesn't actually test anything. The same code already gets accepted by today's Miri.

[LaneAsade]

No worries! I was actually confused by UnsafeCell and UnsafePinned because it seemed like there weren't any differences between them. I genuinely thought that was how it was. Maybe its cuz I couldn't find a lot of information regarding UnsafePinned. I would love to work on this once #152946 is done (Of course, only after I have a full understanding of miri internals). Also, sorry for the trouble you went through to review my PR.

[RalfJung]

I was actually confused by UnsafeCell and UnsafePinned because it seemed like there weren't any differences between them.

As a general rule, please always point out your confusions when submitting a PR. The people reviewing your PRs will thank you. :)

[LaneAsade]

Yes, I will keep that in mind from now on. Thank you for the advice. Again, I apologize for any inconvenience caused.

[RalfJung]

No worries. :)