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