Make settings::Flags::new consume the Builder.

This makes it more clear what the relationship is between the Builder
and the resulting Flags.

lib/frontend/src/frontend.rs

@@ -688,7 +688,7 @@ mod tests {
             builder.finalize();
         }
 
-        let flags = settings::Flags::new(&settings::builder());
+        let flags = settings::Flags::new(settings::builder());
         let res = verify_function(&func, &flags);
         // println!("{}", func.display(None));
         match res {