libstdc++: assert that type traits are not misused with incomplete types [PR 71579]

libstdc++: assert that type traits are not misused with incomplete types [PR 71579]

libstdc++-v3/ChangeLog:

2020-08-19  Antony Polukhin  <antoshkka@gmail.com>

	PR libstdc++/71579
	* include/std/type_traits (invoke_result, is_nothrow_invocable_r)
	Add static_asserts to make sure that the argument of the type
	trait is not misused with incomplete types.
	(is_swappable_with, is_nothrow_swappable_with): Add static_asserts
	to make sure that the first and second arguments of the type trait
	are not misused with incomplete types.
	* testsuite/20_util/invoke_result/incomplete_neg.cc: New test.
	* testsuite/20_util/is_nothrow_invocable/incomplete_neg.cc: New test.
	* testsuite/20_util/is_nothrow_swappable/incomplete_neg.cc: New test.
	* testsuite/20_util/is_nothrow_swappable_with/incomplete_neg.cc: New
	test.
	* testsuite/20_util/is_swappable_with/incomplete_neg.cc: New test.