pgsql: Ensure all pg_locale.h APIs work with collate_is_c.

Ensure all pg_locale.h APIs work with collate_is_c.

Safer for callers, so that checking collate_is_c is only needed if the
caller wants to optimize for that case.

Backpatch to avoid creating a hazard for other backpatches in this
area.

Suggested-by: Andres Freund <andres(at)anarazel(dot)de>
Suggested-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Discussion: https://postgr.es/m/v36ssaygf7grb3qzfsjhtdzi7kqd45ds56nyuf7gi5qjml4qbb@ezmfqzmhlrs2
Backpatch-through: 18

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5e9d23f98cc1c5a7b4add5b843a564fe9c35c6b3

Modified Files
--------------
src/backend/utils/adt/pg_locale.c | 55 ++++++++++++++++++++++++++++++++++-----
1 file changed, 49 insertions(+), 6 deletions(-)