List<CTEClause> asExecutionOrder() {
      List<CTEClause> execution = new ArrayList<CTEClause>();
      asExecutionOrder(new HashSet<CTEClause>(), execution);
      return execution;
    }
Goes though the tabref tree and finds the alias for the table. Once found,
it records the table name-> alias association in aliasToTabs. It also makes
an association from the alias to the table AST in parse info.

@return the alias of the table
ql/src/java/org/apache/hadoop/hive/ql/parse/SemanticAnalyzer.java