Fix ORDER BY syntax

[zier-one]

Index Issue: pingcap/tidb#11486

These are bad (incompatible) SQL cases:

ALTER TABLE t_n ORDER BY ident

Claim this subtask based on the comments below this issue.

[zuoRambo]

Let me fix it

[zuoRambo]

I have some confusions.

1. In Mysql 8.0 Alter Table Doc: Alter Table Syntax is | ORDER BY col_name [, col_name] ..., which means it just need col_name after order by.
   

2. But in Mysql yacc file Order by statement in ALTER TABLE . It has opt_ordering_direction(ASC|DESC).
   

So i think there must something wrong.

[kennytm]

The doc is not always complete. Let's also support the ordering direction.

ALTER TABLE t ORDER BY a ASC, b DESC, c;