Support assign DEFAULT in ON DUPLICATE KEY UPDATE statement

[Deardrops]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t1 (a int unique, 
	b int generated always as (-a) virtual,
	c int generated always as (-a) stored);


insert into t1 values (1,default,default);

insert into t1 values (1,default,default) on duplicate key update a=2, b=default;

2. What did you expect to see?

Query OK, 1 row affected (0.01 sec)

3. What did you see instead?

ERROR 3105 (HY000): The value specified for generated column 'b' in table 't1' is not allowed.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v4.0.0-alpha-693-gada724a2a