clean up create|alter domain stmt incompatiable constraint error case and add regression test

hi.

https://coverage.postgresql.org/src/backend/commands/typecmds.c.gcov.html
show DefineDomain has poor coverage tests.
so I added a more extensive regression test.

also
create domain d_fail as int4 constraint cc GENERATED ALWAYS AS (2) STORED;
ERROR: unrecognized constraint subtype: 4

This kind of error message is not helpful. so I changed it to
ERROR: generated columns are not supported on domains

create domain d_int as int4;
ALTER DOMAIN d_int ADD constraint cc check(value > 1) no inherit not valid;
should fail.
so I made it fail.

typedef struct AlterDomainStmt
{
NodeTag type;
char subtype;
List *typeName; /* domain to work on */
char *name; /* column or constraint name to act on */
Node *def; /* definition of default or constraint */
DropBehavior behavior; /* RESTRICT or CASCADE for DROP cases */
bool missing_ok; /* skip error if missing? */
} AlterDomainStmt;
we only have one `Node *def; ` in AlterDomainStmt
unlike CreateDomainStmt, have `List *constraints;`

so I guess we have to allow the following ALTER DOMAIN queries.
even though the corresponding CREATE DOMAIN stmt would fail.

create domain d_int as int4;
ALTER DOMAIN d_int ADD constraint cc1 check(value > 1) INITIALLY IMMEDIATE; --ok
ALTER DOMAIN d_int ADD constraint cc2 check(value > 1) NOT DEFERRABLE; --ok
ALTER DOMAIN d_int ADD constraint cc3 check(value > 1) NOT DEFERRABLE
not valid; --ok
ALTER DOMAIN d_int ADD constraint cc4 check(value > 1) INITIALLY
IMMEDIATE not valid; --ok

looking at pg_constraint, for the above queries, column {condeferrable
| condeferred | convalidated} all set properly.

i didn't found a way to trigger
errmsg("exclusion constraints not possible for domains")));