libstdc++: Optimize std::destructible concept

This uses variable templates and constraints to define a much simpler
std::destructible concept. This avoids instantiating the trait
std::is_nothrow_destructible and all its implementation in terms of
__is_destructible_safe and __is_destructible_impl.

If we had an intrinsic we could just use that (PR c++/107600).

libstdc++-v3/ChangeLog:

	* include/std/concepts (__detail::__destructible_impl)
	(__detail::__destructible): New variable templates.
	(destructible): Use __detail::__destructible.
	* testsuite/std/concepts/concepts.lang/concept.destructible/1.cc:
	Add more checks for array and reference types.