sql: b'xxxx' should be parsed as a BIT(n)

[mgartner]

In Postgres, a literal like b'1001' is parsed as a BIT(4):

marcus=# SELECT pg_typeof(b'1001');
 pg_typeof
-----------
 bit
(1 row)

We know it is not parsed as a VARBIT because the output is bit, instead of bit varying.

In CRDB v21.2.3, it is parsed as a BYTES:

defaultdb> SELECT pg_typeof(b'1001');
  pg_typeof
-------------
  bytea
(1 row)

We should match Postgres for consistency.

Jira issue: CRDB-12158

[rafiss]

This seems like a dupe of #26128 but it probably is worth keeping this finer-grained issue. The backwards-incompatibility story on fixing this is rough.