libstdc++: Adjust static assertions in futures and promises [LWG 3466]

libstdc++: Adjust static assertions in futures and promises [LWG 3466]

Add a static_assertions to check the result type is destructible, as in
the proposed resolution for LWG 3466 (which supersedes 3458).

libstdc++-v3/ChangeLog:

	* include/std/future (future, shared_future. promise): Add
	is_destructible assertion (LWG 3466). Adjust string-literal for
	!is_array and !is_function assertions.
	* testsuite/30_threads/future/requirements/lwg3458.cc: Check
	types with no accessible destructor. Adjust expected errors.
	* testsuite/30_threads/promise/requirements/lwg3466.cc:
	Likewise.
	* testsuite/30_threads/shared_future/requirements/lwg3458.cc:
	Likewise.