pgsql: Fix t_isspace(), etc., when datlocprovider=i and datctype=C.

Fix t_isspace(), etc., when datlocprovider=i and datctype=C.

Check whether the datctype is C to determine whether t_isspace() and
related functions use isspace() or iswspace().

Previously, t_isspace() checked whether the database default collation
was C; which is incorrect when the default collation uses the ICU
provider.

Discussion: https://postgr.es/m/79e4354d9eccfdb00483146a6b9f6295202e7890.camel@j-davis.com
Reviewed-by: Peter Eisentraut
Backpatch-through: 15

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8b87e92919cd0e8e8ffbae543d996063149c3ccc

Modified Files
--------------
src/backend/tsearch/ts_locale.c | 15 +++++----------
src/backend/tsearch/wparser_def.c | 3 +--
src/backend/utils/adt/pg_locale.c | 3 +++
src/backend/utils/init/postinit.c | 4 ++++
src/include/utils/pg_locale.h | 2 ++
5 files changed, 15 insertions(+), 12 deletions(-)