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 <[email protected]>
Reviewed-by: Matheus Alcantara <[email protected]>
Discussion: 
https://postgr.es/m/cacjufxh3vetr7orf5rw29gndk3n1wwboe3wdkhyd3ipgrq9...@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f80bedd52b1501de51b7180cd817e3e1fc31429d

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml               |  3 ++
src/backend/commands/tablecmds.c                | 25 ++++------------
src/test/regress/expected/generated_virtual.out | 39 +++++++++++++++++++++----
src/test/regress/sql/generated_virtual.sql      | 16 ++++++++--
4 files changed, 56 insertions(+), 27 deletions(-)

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