Added explicit optimization flag to test

Author: dshynkev

src/test/codegen/issue-13018.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-flags: -O
+
 // A drop([...].clone()) sequence on an Rc should be a no-op
 // In particular, no call to __rust_dealloc should be emitted
 #![crate_type = "lib"]