NFC.

The patch fixes r321395, that cuased
 -Werror=unused-but-set-variable issue for
 Diagnosed var on prod build.

From: Evgeny Stupachenko <evstupac@gmail.com>
llvm-svn: 321854

clang/lib/Serialization/ASTReader.cpp

@@ -10660,6 +10660,7 @@ void ASTReader::diagnoseOdrViolations() {
       Diagnosed = true;
       break;
     }
+    (void)Diagnosed;
     assert(Diagnosed && "Unable to emit ODR diagnostic.");
   }
 }