Use stack-allocated StringInfoDatas, where possible

Use stack-allocated StringInfoDatas, where possible

6d0eba662 already did most of the changes, but some new ones snuck in
just prior to that commit, so these got missed.

Having these short-lived StringInfoDatas on the stack rather than having
them get palloc'd by makeStringInfo() is simply for performance as it
saves doing a 2nd palloc.

Since this code is new to v19, it makes sense to improve it now rather
than wait until we branch as having v19 and v20 differ here just makes it
harder to backpatch fixes in this area.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/adt4wpj4FZwR+S7I@ip-10-97-1-34.eu-west-3.compute.internal