fix warning towards clang7

warning: binding reference member 'n1_' to stack allocated parameter
'n1'

Change-Id: Ifa4c1520453e00d6fc35411fa0edbf2bf2b42708

compilerplugins/clang/test/casttovoid.cxx

@@ -57,8 +57,8 @@ struct S1 {
         (void) n1; // expected-error {{unnecessary cast to void [loplugin:casttovoid]}}
         (void) n2; // expected-error {{unnecessary cast to void [loplugin:casttovoid]}}
     }
-    int const & n1_;
-    int const & n2_;
+    int const n1_;
+    int const n2_;
 };
 
 struct S2 { int n; };