Auto merge of #112062 - lukas-code:unsized-layout, r=wesleywiser

Make struct layout not depend on unsizeable tail

fixes (after backport) rust-lang/rust#112048

Since unsizing `Ptr<Foo<T>>` -> `Ptr<Foo<U>` just copies the pointer and adds the metadata, the layout of `Foo` must not depend on niches in and alignment of the tail `T`.

Nominating for beta 1.71, because it will have this issue: `@rustbot` label beta-nominated