Refactor to introduce pg_locale_deterministic().
Avoids the need of callers to test for NULL, and also avoids the need
to access the pg_locale_t structure directly.
Reviewed-by: Peter Eisentraut, Peter Geoghegan
Discussion: https://postgr.es/m/
a581136455c940d7bd0ff482d3a2bd51af25a94f.camel%40j-davis.com