SGTools: Move static variable out-of-line.

SGTools: Move static variable out-of-line.

Move the empty string variable used in TransientAddress::name()
from being defined in that inlined method to a static class member
which is instead defined out-of-line.

Avoids crashes seen related to library loading ordering when compiled with LTO.

Control/SGTools/SGTools/TransientAddress.h

 /*
-  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef SGTOOLS_TRANSIENTADDRESS_H
@@ -175,6 +175,8 @@ namespace SG {
 
     ///< all alias names for a DataObject. They come from setAlias
     TransientAliasSet m_transientAlias;
+
+    static const std::string s_emptyString;
   };
   /////////////////////////////////////////////////////////////////////
   // inlined code:
@@ -212,8 +214,7 @@ namespace SG {
     if (m_name.isValid()) {
       return *m_name.ptr();
     }
-    static const std::string empty;
-    return empty;
+    return s_emptyString;
   }
 
   /// Get the primary (hashed) SG key.

Control/SGTools/src/TransientAddress.cxx

 /*
-  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "SGTools/TransientAddress.h"
@@ -12,6 +12,8 @@
 
 using SG::TransientAddress;
 
+const std::string TransientAddress::s_emptyString;
+
 // Default Constructor:
 TransientAddress::TransientAddress()
   : TransientAddress (CLID_NULL, "", nullptr, true, false)