Fix under-parenthesized display of AT TIME ZONE constructs.

In commit 40c24bfef, I forgot to use get_rule_expr_paren() for the
arguments of AT TIME ZONE, resulting in possibly not printing parens
for expressions that need it.  But get_rule_expr_paren() wouldn't have
gotten it right anyway, because isSimpleNode() hadn't been taught that
COERCE_SQL_SYNTAX parent nodes don't guarantee sufficient parentheses.
Improve all that.  Also use this methodology for F_IS_NORMALIZED, so
that we don't print useless parens for that.

In passing, remove a comment that was obsoleted later.

Per report from Duncan Sands.  Back-patch to v14 where this code
came in.  (Before that, we didn't try to print AT TIME ZONE that way,
so there was no bug just ugliness.)

Discussion: https://postgr.es/m/f41566aa-a057-6628-4b7c-b48770ecb84a@deepbluecap.com

src/test/regress/sql/create_view.sql

@@ -703,6 +703,7 @@ select pg_get_viewdef('tt20v', true);
 
 create view tt201v as
 select
+  ('2022-12-01'::date + '1 day'::interval) at time zone 'UTC' as atz,
   extract(day from now()) as extr,
   (now(), '1 day'::interval) overlaps
     (current_timestamp(2), '1 day'::interval) as o,