Add parse location to IndexElem.

Add parse location to IndexElem.

This patch mostly just fills in the field, although a few error
reports in resolve_unique_index_expr() are adjusted to use it.
The next commit will add more uses.

catversion bump out of an abundance of caution: I'm not sure
IndexElem can appear in stored rules, but I'm not sure it can't
either.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Co-authored-by: jian he <jian.universality@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CACJufxH3OgXF1hrzGAaWyNtye2jHEmk9JbtrtGv-KJK6tsGo5w@mail.gmail.com
Discussion: https://postgr.es/m/202512121327.f2zimsr6guso@alvherre.pgsql