Re: ALTER COLUMN ... SET EXPRESSION to alter stored generated column's expression

Here is the rebase version for the latest master head(673a17e3120).

I haven't done any other changes related to the ON UPDATE trigger since that
seems non-trivial; need a bit of work to add trigger support in ATRewriteTable(). 

Also, I am not sure yet, if we were doing these changes, and the correct direction

for that.

Regards,

Amul