Allow ALTER COLUMN SET EXPRESSION on virtual columns with CHECK constraints

Allow ALTER COLUMN SET EXPRESSION on virtual columns with CHECK constraints

Previously, changing the generation expression of a virtual column was
prohibited if the column was referenced by a CHECK constraint.  This
lifts that restriction.

RememberAllDependentForRebuilding within ATExecSetExpression will
rebuild all the dependent constraints, later ATPostAlterTypeCleanup
queues the required AlterTableStmt operations for ALTER TABLE Phase 3
execution.

Overall, ALTER COLUMN SET EXPRESSION on virtual columns may require
scanning the table to re-verify any associated CHECK constraints, but
it does not require a table rewrite in ALTER TABLE Phase 3.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Discussion: https://postgr.es/m/CACJufxH3VETr7orF5rW29GnDk3n1wWbOE3WdkHYd3iPGrQ9E_A@mail.gmail.com