Rollup merge of #92704 - 5225225:std_mem_transmute_ref_t_mut_t, r=michaelwoerister

Change lint message to be stronger for &T -> &mut T transmute

The old message implied that it's only UB if you use the reference to mutate, which (as far as I know) is not true. As in, the following program has UB, and a &T -> &mut T transmute is effectively an `unreachable_unchecked`.

```rust
fn main() {
    #[allow(mutable_transmutes)]
    unsafe {
        let _ = std::mem::transmute::<&i32, &mut i32>(&0);
    }
}
```

In the future, it might be a good idea to use the edition system to make this a hard error, since I don't think it is *ever* defined behaviour? Unless we rule that `&UnsafeCell<i32> -> &mut i32` is fine. (That, and you always could just use `.get()`, so you're not losing anything)

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