Boolean partitions syntax

Hi.

Horiguchi-san pointed out [1] on a nearby thread that the partitioning
syntax (the FOR VALUES clause) doesn't accept true and false as valid
partition bound datums, which seems to me like an oversight.  Attached a
patch to fix that.

create table bools (a bool) partition by list (a);

Before patch:

create table bools_t partition of bools for values in (true);
ERROR:  syntax error at or near "true"
LINE 1: ...reate table bools_t partition of bools for values in (true);

After:

create table bools_t partition of bools for values in (true);
CREATE TABLE
\d bools_t
              Table "public.bools_t"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | boolean |           |          |
Partition of: bools FOR VALUES IN (true)

Thanks,
Amit

[1]
https://www.postgresql.org/message-id/20171128.203915.26713586.horiguchi.kyotaro%40lab.ntt.co.jp