code: private static SubstUnitInfo genSubstUnitInfo(MsgNode msgNode, ErrorReporter errorReporter) {
      return genFinalSubstUnitInfoMapsHelper(
          RepresentativeNodes.createFromNode(msgNode, errorReporter));
    }
docs: Helper function to generate SubstUnitInfo, which contains mappings from/to substitution unit
  nodes (placeholders and plural/select nodes) to/from generated var names.
  
  <p>It is guaranteed that the same var name will never be shared by multiple nodes of different
  types (types are placeholder, plural, and select).
  
  @param msgNode The MsgNode to process.
  @return The generated SubstUnitInfo for the given MsgNode.