Miri does not properly check `dereferenceable` on `&mut !Unpin`

[RalfJung]

#2713 works around rust-lang/unsafe-code-guidelines#381 by just making Miri check fewer things, to avoid blocking people that gate their CI on Miri. This almost certainly means Miri currently misses some LLVM UB. So while the UCG discusses how the issue is to be resolved, let's track on the Miri side that we are missing some UB here. (#2713 removes a fail test that should possibly be reinstated once we figure out what to do here, or maybe be turned into a pass test.)