Fix incremental tests

Author: varkor

src/test/incremental/change_crate_order/main.rs

@@ -20,5 +20,5 @@ use b::B;
 
 //? #[rustc_clean(label="typeck_tables_of", cfg="rpass2")]
 pub fn main() {
-    A + B;
+    let _ = A + B;
 }

src/test/incremental/warnings-reemitted.rs

@@ -6,5 +6,5 @@
 #![warn(const_err)]
 
 fn main() {
-    255u8 + 1; //~ WARNING this expression will panic at run-time
+    let _ = 255u8 + 1; //~ WARNING this expression will panic at run-time
 }