Allow shortening lifetime in CoerceUnsized for &mut
This modifies the &mut -> &mut CoerceUnsized impl so that, it can
shorten the lifetime.
Note that there are already two impls that allow shortening the lifetime
like this (the &mut T -> &U and the &T -> &U impls). So this change
makes the impls consistent with each other.
I initially tried to also do the same to the CoerceUnsized impl for
core::cell::{Ref, RefMut}. However, this can't be done because
Ref and RefMut "store" the lifetime and the data in different fields,
and CoerceUnsized can only coerce one field.
This change has an effect on stable code, since it allows shortening
lifetimes inside invariant types via a &mut -> &mut unsize coercion.
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