Rollup merge of rust-lang#133784 - dtolnay:visitspans, r=compiler-errors

Fix MutVisitor's default implementations to visit Stmt's and BinOp's spans

The `Stmt` case is a bug introduced almost certainly unintentionally by rust-lang#126993. The code _used_ to visit and mutate `span` correctly, but got changed as follows by that PR. Notice how `span` is **copied** into the output by `|kind| Stmt { id, kind, span }` which happens after the mutation in the correct code (red) and before the mutation in the incorrect code (green).

```diff
  pub fn noop_flat_map_stmt<T: MutVisitor>(
      Stmt { kind, mut span, mut id }: Stmt,
      vis: &mut T,
  ) -> SmallVec<[Stmt; 1]> {
      vis.visit_id(&mut id);
-     vis.visit_span(&mut span);
      let stmts: SmallVec<_> = noop_flat_map_stmt_kind(kind, vis)
          .into_iter()
          .map(|kind| Stmt { id, kind, span })
          .collect();
      if stmts.len() > 1 {
          panic!(...);
      }
+     vis.visit_span(&mut span);
      stmts
  }
```