pgsql: Allow ALTER COLUMN SET EXPRESSION on virtual columns with CHECK

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(dot)universality(at)gmail(dot)com>
Reviewed-by: Matheus Alcantara <matheusssilv97(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACJufxH3VETr7orF5rW29GnDk3n1wWbOE3WdkHYd3iPGrQ9E_A@mail.gmail.com

Branch
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master

Details
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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(-)