bug in stored generated column over domain with constraints.

hi.

create domain d1 as int not null;
create domain d2 as int check (value > 1);
create domain d3 as int check (value is not null);

create table t0(b int, a d3 GENERATED ALWAYS as (b + 11) stored);
insert into t0 values (1, default);
ERROR:  value for domain d3 violates check constraint "d3_check"

in the above example, a domain with check constraint not working as intended,
similarly, a domain with not-null constraint will also not work.
now table t0 can not insert any data, because of check constraint violation.
so i think this is a bug, (hope i didn't miss anything).

in ExecBuildProjectionInfo, we compile "values (1, default)"  as
targetlist expression (CONST, COERCETODOMAIN)
Then in ExecResult, ExecProject, ExecInterpExpr we evaluate the
compiled expression;
we failed at ExecEvalConstraintCheck. we are acting like: ``null::d3``.

explain(costs off, verbose) insert into t0 values (1, default);
               QUERY PLAN
----------------------------------------
 Insert on public.t0
   ->  Result
         Output: 1, NULL::integer

the plan is ``NULL::integer``, which will not fail, but  ``NULL::d3`` will fail.
that means, the output plan is right. it's the execution wrong?


the main fix should be in rewriteTargetListIU.
UPDATE don't have this issue, since there is no Result node,
ExecComputeStoredGenerated will do the domain constraint check.

related:
https://git.postgresql.org/cgit/postgresql.git/commit/?id=0da39aa7667b06e16189d318f7850d559d446d52