emit Retag for compound types with reference fields
I want to add an option to Miri to do retagging inside reference fields. But that means we first have to even emit `Retag` for types that *contain* references (rather than being of reference types). :)
Stacked Borrows originally did that, but we stopped doing it when hitting bunch of issues in the standard library. However I have since realized that we actually do emit `noalias` for newtypes references, which means for soundness we should recurse into fields. Also it'd probably be bad news if newtypes lose out on optimizations (and they don't, for anything else). I want to add an option for that to Miri so that we can start experimenting with those semantics.
r? ``@oli-obk``
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