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For the former, it's fine for `inbounds` offsets to be one-past-the-end,
so it's okay even if the ZST is the last field in the layout:
> The base pointer has an in bounds address of an allocated object,
> which means that it points into an allocated object, or to its end.
https://llvm.org/docs/LangRef.html#getelementptr-instruction
For the latter, even DST fields must always be inside the layout
(or to its end for ZSTs), so using inbounds is also fine there.
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Previously, it was only put on scalars with range validity invariants
like bool, was uninit was obviously invalid for those.
Since then, we have normatively declared all uninit primitives to be
undefined behavior and can therefore put `noundef` on them.
The remaining concern was the `mem::uninitialized` function, which cause
quite a lot of UB in the older parts of the ecosystem. This function now
doesn't return uninit values anymore, making users of it safe from this
change.
The only real sources of UB where people could encounter uninit
primitives are `MaybeUninit::uninit().assume_init()`, which has always
be clear in the docs about being UB and from heap allocations (like
reading from the spare capacity of a vec. This is hopefully rare enough
to not break anything.
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