public String getDisplayName() {
    return displayName;
  }
Get the display name for this function. This should be transferred into
exprNodeGenericUDFDesc, and will be used as the first parameter to
GenericUDF.getDisplayName() call, instead of hard-coding the function name.
This will solve the problem of displaying only one name when a udf is
registered under 2 names.
ql/src/java/org/apache/hadoop/hive/ql/exec/FunctionInfo.java