parser: fix alter_stmt order by syntax

[zuoRambo]

What problem does this PR solve?

Fix ORDER BY syntax. Issue: #499

MySQL Syntax:

alter_specification:
...
  | ORDER BY col_name [, col_name] ...

Bad SQL Case:

ALTER TABLE t_n ORDER BY ident

Check List

Tests

• Unit test

Code changes

• Has exported variable/fields change
• Has interface methods change

Related changes

• Need to update the documentation

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #526 into master will decrease coverage by <.01%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
- Coverage   71.77%   71.76%   -0.01%     
==========================================
  Files          32       32              
  Lines        7787     7800      +13     
==========================================
+ Hits         5589     5598       +9     
- Misses       1672     1674       +2     
- Partials      526      528       +2

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)	⬆️
ast/ddl.go	80.44% <69.23%> (-0.1%)	⬇️

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[zier-one]

LGTM

[zier-one]

@tangenta PTAL

[tangenta]

Rest LGTM

[tangenta]

LGTM