Fix misuse of Relids for storing attribute numbers

Fix misuse of Relids for storing attribute numbers

The typedef Relids (Bitmapset *) is intended to represent set of
relation identifiers, but was incorrectly used in several places to
store sets of attribute numbers.  This is my oversight in e2debb643.

Fix that by replacing such usages with Bitmapset * to reflect the
correct semantics.

Author: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAEG8a3LJhp_xriXf39iCz0TsK+M-2biuhDhpLC6Baxw8+ZYT3A@mail.gmail.com