pgsql-server: Fix some bogus code in ConstBit production --- it managed

Log Message:
-----------
Fix some bogus code in ConstBit production --- it managed to work, but
only because 14627 still contained the same node that BitWithoutLength had
just produced.  Make it more transparent.  Also adjust ConstCharacter
to be coded the same way for consistency.

Modified Files:
--------------
    pgsql-server/src/backend/parser:
        gram.y (r2.463 -> r2.464)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/parser/gram.y.diff?r1=2.463&r2=2.464)