Auto merge of #97522 - xfix:stabilize-slice-from-raw-parts, r=dtolnay

Partially stabilize const_slice_from_raw_parts

This doesn't stabilize methods working on mutable pointers.

This pull request continues from #94946.

Pinging `@rust-lang/wg-const-eval` this because I use `rustc_allow_const_fn_unstable`. I believe this is justifiable as it's already possible to use `slice::from_raw_parts` in stable by abusing `transmute`. The stable alternative to this would be to provide a stable const implementation of `std::ptr::from_raw_parts` (as it can already be implemented in stable).

```rust
use std::mem;

#[repr(C)]
struct Slice<T> {
    data: *const T,
    len: usize,
}

fn main() {
    let data: *const i32 = [1, 2, 3, 4].as_ptr();
    let len = 4;
    println!("{:?}", unsafe {
        mem::transmute::<Slice<i32>, &[i32]>(Slice { data, len })
    });
}
```

`@rustbot` modify labels: +T-libs-api

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