message: replace a borrow_mut by get_mut
diff: @@ -935,11 +935,11 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
  // Note that this asserts that the allocation is mutable -- but since we are creating a
  // mutable pointer, that seems reasonable.
  let (alloc_extra, machine) = this.get_alloc_extra_mut(alloc_id)?;
  - let mut stacked_borrows = alloc_extra
  + let stacked_borrows = alloc_extra
  .stacked_borrows
  .as_mut()
  .expect("we should have Stacked Borrows data")
  - .borrow_mut();
  + .get_mut();
  let item = Item::new(new_tag, perm, protect.is_some());
  let range = alloc_range(base_offset, size);
  let global = machine.stacked_borrows.as_ref().unwrap().borrow();
  