Auto merge of #91616 - saethlin:sort_unchecked-sb-fix, r=Mark-Simulacrum

Fix #91306 by deriving all access from a single *mut T

Fixes #91306.

The previous code is invalid because the first argument to `copy_nonoverlapping` is invalidated by the mutable borrow taken out to construct the second argument.

I believe this patch fixes that, and this code should now pass Miri with `-Ztag-raw-pointers`, ~~but I'm currently stuck trying to run my reproducer with a this patched version of the standard library (alternatively, running Miri on the standard library tests itself would suffice).~~ Ralf walked me through this on Zulip.

I've also added fixes for 7 more problems other than those I reported. Most of them are easy to hit by calling sort_unstable on random arrays. I don't have reproducers for every change, but they seem pretty clear-cut to me. But I did only start learning stacked borrows 2 days ago so that might be a large dash of Dunning-Kruger.

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