Re: support virtual generated column not null constraint

hi.

new patch attached.

0001 for virtual generated columns not null.
minor change to fix the compiler warning.
0002-0004 is for domain over virtual generated columns.

0002: we need to compute the generation expression for the domain with
constraints,
thus rename ExecComputeStoredGenerated to ExecComputeGenerated.

0003. preparatory patch for 0004.
soft error variant of ExecPrepareExpr, ExecInitExpr.
for soft error processing of CoerceToDomain. see below description.

0004. no table rewrite for adding virtual generation column over
domain with constraints.
(syntax: ALTER TABLE xx ADD COLUMN X domain_type GENERATED ALWAYS AS
(expression) VIRTUAL
in phase3, ATRewriteTable: we already initialized AlteredTableInfo->newvals via
``ex->exprstate = ExecInitExpr((Expr *) ex->expr, NULL);``
we can easily evaluate it via ExecCheck. if fail, then error out.

--------------
reason for the 0003 patch:

ALTER DOMAIN ADD CONSTRAINT.
since the virtual generated column has no actual storage.
so, in validateDomainCheckConstraint we cannot use
```
d = slot_getattr(slot, attnum, &isNull);
econtext->domainValue_datum = d;
econtext->domainValue_isNull = isNull;
conResult = ExecEvalExprSwitchContext(exprstate,
econtext,
&isNull);

```
to check whether existing generation expression satisfy the newly
added domain constraint or not.

we also need to evaluate error softly,
because
ALTER DOMAIN ADD CONSTRAINT need check exists table domain value satisfy
the newly constraint or not.
if we not do soft error evaluation, the error message would be like:
``value for domain gtestdomain1 violates check constraint "gtestdomain1_check"``
but we want error message like:
ERROR: column "b" of table "gtest24" contains values that violate the
new constraint
--------------