c++: Fix __is_*constructible/assignable for templates [PR102305]

is_xible_helper returns error_mark_node (i.e. false from the traits)
for abstract classes by testing ABSTRACT_CLASS_TYPE_P (to) early.
Unfortunately, as the testcase shows, that doesn't work on class templates
that haven't been instantiated yet, ABSTRACT_CLASS_TYPE_P for them is false
until it is instantiated, which is done when the routine later constructs
a dummy object with that type.

The following patch fixes this by calling complete_type first, so that
ABSTRACT_CLASS_TYPE_P test will work properly, while keeping the handling
of arrays with unknown bounds, or incomplete types where it is done
currently.

2021-09-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102305
	* method.c (is_xible_helper): Call complete_type on to.

	* g++.dg/cpp0x/pr102305.C: New test.