public static void visit(Object t, ContextVisitor visitor) {
    _visit(t, null, 0, visitor);
  }
For each column on the input RR, construct a StructField for it
OI is constructed using the list of input column names and
their corresponding OIs.

@param rr
@return
ql/src/java/org/apache/hadoop/hive/ql/parse/PTFTranslator.java