c++: __is_destructible fixes [PR107600]

c++: __is_destructible fixes [PR107600]

destructible_expr was wrongly assuming that TO is a class type.

When is_xible_helper was added in r8-742 it returned early for abstract
class types, which is correct for __is_constructible, but not
__is_assignable or (now) __is_destructible.

	PR c++/107600

gcc/cp/ChangeLog:

	* method.cc (destructible_expr): Handle non-classes.
	(constructible_expr): Check for abstract class here...
	(is_xible_helper): ...not here.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_destructible2.C: New test.