// Gets the location information of the given node. It uses the
  // "original_node_name" in the NodeDef to get the corresponding file location
  // (FileLineColLoc) from the input DebugInfo and returns an CallSiteLoc. If
  // there are multiple "original_node_names", a FusedLoc is returned. If the
  // node name couldn't be found in the input DebugInfo, a NameLoc is used as
  // the location.