Interior mutable shared ref to constant memory is UB even when not writing

[RalfJung]

Current Stacked Borrows makes this UB:

fn main() { unsafe {
    let x = &0usize;
    let y: &std::cell::Cell<usize> = std::mem::transmute(x);
} }

To be honest I do not think there is a good reason for this. We cannot optimize much around interior mutable types, so I don't think we should require them to be writable unless we are actually mutating.

[RalfJung]

This is closely related to #227: basically, interior mutable shared references should inherit the parent pointer permission instead of asserting mutability, and again we have a close relationship between what we do for raw pointers and what we do for interior mutable shared references.

@programmerjake that's the exact same case. AtomicUsize and Cell<usize> look the same to the compiler (both are UnsafeCell<usize>).

[RalfJung]

Closing as a duplicate of #303.