Replace stray println!() in lint code by bug!()

To avoid crashing Clippy, the `bug!()` is used only when debug assertions are
enabled. In regular usage, the result will be the same as before, but without
the extra line printed on the standard output which has the potential for
disrupting shell scripts.

Author: samueltardieu

clippy_lints/src/arbitrary_source_item_ordering.rs

@@ -382,7 +382,9 @@ impl<'tcx> LateLintPass<'tcx> for ArbitrarySourceItemOrdering {
                         // Filters the auto-included Rust standard library.
                         continue;
                     }
-                    println!("Unknown item: {item:?}");
+                    if cfg!(debug_assertions) {
+                        rustc_middle::bug!("unknown item: {item:?}");
+                    }
                 }
             } else if let ItemKind::Impl(_) = item.kind
                 && get_item_name(item).is_some()