Make clippy set mir_opt_level using Option

src/driver.rs

@@ -83,7 +83,7 @@ fn config(&mut self, config: &mut interface::Config) {
         // run on the unoptimized MIR. On the other hand this results in some false negatives. If
         // MIR passes can be enabled / disabled separately, we should figure out, what passes to
         // use for Clippy.
-        config.opts.debugging_opts.mir_opt_level = 0;
+        config.opts.debugging_opts.mir_opt_level = Some(0);
     }
 }