[cling] Make cling::utils::Lookup::Named look into using directive

Author: devajithvs

interpreter/cling/lib/Utils/AST.cpp

@@ -1501,7 +1501,15 @@ namespace utils {
         // No definition, no lookup result.
         return;
       }
-      S->LookupQualifiedName(R, const_cast<DeclContext*>(primaryWithin));
+      auto res =
+          S->LookupQualifiedName(R, const_cast<DeclContext*>(primaryWithin));
+
+      // If the lookup fails and the context is a namespace, try to lookup in
+      // the namespaces brought in by using directives.
+      if (!res && primaryWithin->isNamespace())
+        for (auto* UD : primaryWithin->using_directives())
+          if (NamespaceDecl* NS = UD->getNominatedNamespace())
+            res = S->LookupQualifiedName(R, NS);
     }
   }