xAODRootAccess: Fix cppyy nodiscard warnings.

cppyy will generate wrappers both for functions returning a value
and the value being discarded.  If we have [[nodiscard]] on,
then we'll get warnings when cling compiles these wrappers.
Since these wrappers are automatically generated by cppyy,
we can't really avoid these other than by disabling them
for the cling case.

Control/xAODRootAccess/xAODRootAccess/tools/TReturnCode.h

 // Dear emacs, this is -*- c++ -*-
 
 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 
 // $Id: TReturnCode.h 598368 2014-05-22 10:23:54Z krasznaa $
@@ -27,7 +27,17 @@ namespace xAOD {
    /// $Revision: 598368 $
    /// $Date: 2014-05-22 12:23:54 +0200 (Thu, 22 May 2014) $
    ///
-   class [[nodiscard]] TReturnCode {
+   class
+#ifndef __CLING__
+   // cppyy will generate wrappers both for functions returning a value
+   // and the value being discarded.  If we have [[nodiscard]] on,
+   // then we'll get warnings when cling compiles these wrappers.
+   // Since these wrappers are automatically generated by cppyy,
+   // we can't really avoid these other than by disabling them
+   // for the cling case.
+   [[nodiscard]]
+#endif
+   TReturnCode {
 
    public:
       /// Enumeration listing the possible return codes