Rollup merge of #81479 - osa1:issue24151, r=lcnr

Rollup merge of #81479 - osa1:issue24151, r=lcnr

Allow casting mut array ref to mut ptr

Allow casting mut array ref to mut ptr

We now allow two new casts:

- mut array reference to mut ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *mut usize;

  We allow casting const array references to const pointers so not
  allowing mut references to mut pointers was inconsistent.

- mut array reference to const ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *const usize;

  This was similarly inconsistent as we allow casting mut references to
  const pointers.

Existing test 'vector-cast-weirdness' updated to test both cases.

Fixes #24151