This patch adds static asserts for type traits misuse with incomplete
classes and unions. This gives a nice readable error message instead
of an UB and odr-violations.

Some features of the patch:
* each type trait has it's own static_assert inside. This gives better
diagnostics than the approach with putting the assert into a helper
structure and using it in each trait.
* the result of completeness check is not memorized by the compiler.
This gives no false positive after the first failed check.
* some of the compiler builtins already implement the check. But not
all of them! So the asserts are in all the type_traits that may
benefit from the check. This also makes the behavior of libstdc++ more
consistent across different (non GCC) compilers.
* std::is_base_of does not have the assert as it works well in many
cases with incomplete types

2019-05-31  Antony Polukhin  <antoshkka@gmail.com>

	PR libstdc++/71579
	* include/std/type_traits __type_identity, __is_complete_or_unbounded):
	New helpers for checking preconditions in traits.
	(is_trivial, is_trivially_copyable, is_standard_layout, is_pod)
	(is_literal_type, is_empty, is_polymorphic, is_final, is_abstract)
	(is_destructible, is_nothrow_destructible, is_constructible)
	(is_default_constructible, is_copy_constructible)
	(is_move_constructible, is_nothrow_default_constructible)
	(is_nothrow_constructible, is_nothrow_copy_constructible)
	(is_nothrow_move_constructible, is_copy_assignable, is_move_assignable)
	(is_nothrow_assignable, is_nothrow_copy_assignable)
	(is_nothrow_move_assignable, is_trivially_constructible)
	(is_trivially_copy_constructible, is_trivially_move_constructible)
	is_trivially_assignable, is_trivially_copy_assignable)
	(is_trivially_move_assignable, is_trivially_destructible)
	(alignment_of, is_swappable, is_nothrow_swappable, is_invocable)
	(is_invocable_r, is_nothrow_invocable)
	(has_unique_object_representations, is_aggregate): Add static_asserts
	to make sure that type traits are not misused with incomplete types.
	(__is_constructible_impl, __is_nothrow_default_constructible_impl)
	(__is_nothrow_constructible_impl, __is_nothrow_assignable_impl): New
	base characteristics without assertions that can be reused in other
	traits.
	* testsuite/20_util/is_complete_or_unbounded/memoization.cc: New test.
	* testsuite/20_util/is_complete_or_unbounded/memoization_neg.cc: New
	test.
	* testsuite/20_util/is_complete_or_unbounded/value.cc: New test.
	* testsuite/20_util/is_abstract/incomplete_neg.cc: New test.
	* testsuite/20_util/is_aggregate/incomplete_neg.cc: New test.
	* testsuite/20_util/is_class/value.cc: Check incomplete type.
	* testsuite/20_util/is_function/value.cc: Likewise.
	* testsuite/20_util/is_move_constructible/incomplete_neg.cc: New test.
	* testsuite/20_util/is_nothrow_move_assignable/incomplete_neg.cc: New
	test.
	* testsuite/20_util/is_polymorphic/incomplete_neg.cc: New test.
	* testsuite/20_util/is_reference/value.cc: Check incomplete types.
	* testsuite/20_util/is_unbounded_array/value.cc: Likewise.
	* testsuite/20_util/is_union/value.cc: Likewise.
	* testsuite/20_util/is_void/value.cc: Likewise.
	* testsuite/util/testsuite_tr1.h: Add incomplete union type.

From-SVN: r271806