Auto merge of #97027 - cuviper:yesalias-refcell, r=thomcc

Auto merge of #97027 - cuviper:yesalias-refcell, r=thomcc

Use pointers in `cell::{Ref,RefMut}` to avoid `noalias`

When `Ref` and `RefMut` were based on references, they would get LLVM `noalias` attributes that were incorrect, because that alias guarantee is only true until the guard drops. A `&RefCell` on the same value can get a new borrow that aliases the previous guard, possibly leading to miscompilation. Using `NonNull` pointers in `Ref` and `RefCell` avoids `noalias`.

Fixes the library side of #63787, but we still might want to explore language solutions there.