void asExecutionOrder(Set<CTEClause> visited, List<CTEClause> execution) {
      for (CTEClause parent : parents) {
        if (visited.add(parent)) {
          parent.asExecutionOrder(visited, execution);
        }
      }
      execution.add(this);
    }
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