support ALTER COLUMN SET EXPRESSION over virtual generated column with check constraint

hi.

in RememberAllDependentForRebuilding
while (HeapTupleIsValid(depTup = systable_getnext(scan)))
{
if(subtype == AT_SetExpression)
elog(INFO, "foundObject.classId:%d", foundObject.classId);
}
Then do the regress test on generated_stored.sql
I found out only constraints and indexes will be rebuilt
while we are doing ALTER TABLE ALTER COLUMN SET EXPRESSION.

we can also see RememberAllDependentForRebuilding handling of:
case RelationRelationId:
case AttrDefaultRelationId:
case ConstraintRelationId:

RememberAllDependentForRebuilding record
AlteredTableInfo->changedConstraintOids, AlteredTableInfo->changedIndexOids.
ATPostAlterTypeCleanup will construct steps to rebuild these
constraints over the generated column.
and if these constraints are successfully installed,
AlteredTableInfo->constraints will be populated.
then in phase3 ATRewriteTable will do the scan or rewrite.

in summary: ATExecSetExpression, RememberAllDependentForRebuilding
will do all the work to change the generation expression,
whether it's virtual or stored.

we didn't support virtual generated columns over domain with check constraint.
we also didn't support index over virtual generated columns.
to support change generation expressions for virtual generated columns
over check constraints,
the code seems not hard.