support fast default for domain with constraints

hi.

Thanks to commit aaaf9449ec6be62cb0d30ed3588dc384f56274bf[1],
ExprState.escontext (ErrorSaveContext) was added, and ExecEvalConstraintNotNull,
ExecEvalConstraintCheck were changed to use errsave instead of hard error.
Now we can use it to evaluate CoerceToDomain in a soft error way, that
is what this patch intended to do.
previously ExprState.escontext was mainly used in SQL/JSON related patches.

To achieve that, we have to populate ExprState.escontext before
passing it to ExecInitExprRec.
So I created two functions: ExecInitExprSafe, ExecPrepareExprSafe.
ExecPrepareExprSafe is an error safe variant of ExecPrepareExpr.
within ExecPrepareExprSafe, we use ExecInitExprSafe.
ExecInitExprSafe differs from ExecInitExpr is that the output
ExprState has its escontext set to a valid ErrorSaveContext.

demo:
CREATE DOMAIN domain5 AS int check(value > 10); -- stable
create domain domain6 as int not null;

CREATE TABLE t3(a int);
ALTER TABLE t3 ADD COLUMN b domain5 default 1; --should not fail.
INSERT INTO t3 DEFAULT VALUES; --should fail.
ALTER TABLE t3 DROP COLUMN b; --need drop it for the following tests
INSERT INTO t3 VALUES(1),(2);

ALTER TABLE t3 ADD COLUMN b domain6; --table rewrite. then fail.
ALTER TABLE t3 ADD COLUMN c domain6 default 13; --no table rewrite.
fast default applied. attmissingval is stored.

[1] https://git.postgresql.org/cgit/postgresql.git/commit/?id=aaaf9449ec6be62cb0d30ed3588dc384f56274bf