work around deviation in diagnostic output for opt-level=0 by forcing -O.

Author: pnkfelix

src/test/ui/lint/lint-type-overflow2.rs

@@ -7,8 +7,8 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
-//
 
+// compile-flags: -O
 #![warn(overflowing_literals)]
 #![warn(const_err)]
 // compile-pass