Change one loop in ATRewriteTable to use 1-based attnums

Change one loop in ATRewriteTable to use 1-based attnums

All TupleDescAttr() calls in tablecmds.c that aren't in loops across all
attributes use AttrNumber-style indexes (1-based); there was only one
place in ATRewriteTable that was stashing 0-based indexes in a list for
later processing.  Switch that to use attnums for consistency.

Author: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxEoYA5ScUr2=CmA1xcpaS_1ixneDbEkVU77X1ctGxY2mA@mail.gmail.com