Xcode: Fix nested source group handling (#12943)

Teach the code path for nested sources build the group map key in the
same way as non-nested code path does.

Source/cmGlobalXCodeGenerator.cxx

@@ -2914,7 +2914,7 @@ cmXCodeObject* cmGlobalXCodeGenerator
     {
     std::vector<std::string> folders =
       cmSystemTools::tokenize(sg->GetFullName(), "\\");
-    cmStdString curr_folder = cmtarget.GetName();
+    cmStdString curr_folder = target;
     curr_folder += "/";
     for(std::vector<std::string>::size_type i = 0; i < folders.size();i++)
       {