This implements the new requirements for C++20 that std::atomic should
initialize the atomic variable in its default constructor.

This patch does not add the deprecated attribute to atomic_init, but
that should be done at some point as it's deprecated in C++20.

The paper also deprecates the ATOMIC_FLAG_INIT macro, although we can't
apply the deprecated attribute to a macro.

	PR libstdc++/58605
	* include/bits/atomic_base.h (__cpp_lib_atomic_value_initialization):
	Define.
	(__atomic_flag_base, __atomic_base, __atomic_base<_PTp*>)
	(__atomic_float): Add default member initializer for C++20.
	* include/std/atomic (atomic): Likewise.
	(atomic::atomic()): Remove noexcept-specifier on default constructor.
	* include/std/version (__cpp_lib_atomic_value_initialization): Define.
	* testsuite/29_atomics/atomic/cons/assign_neg.cc: Adjust dg-error line
	number.
	* testsuite/29_atomics/atomic/cons/copy_neg.cc: Likewise.
	* testsuite/29_atomics/atomic/cons/value_init.cc: New test.
	* testsuite/29_atomics/atomic_flag/cons/value_init.cc: New test.
	* testsuite/29_atomics/atomic_flag/requirements/trivial.cc: Adjust
	expected result for is_trivially_default_constructible.
	* testsuite/29_atomics/atomic_float/requirements.cc: Likewise.
	* testsuite/29_atomics/atomic_float/value_init.cc: New test.
	* testsuite/29_atomics/atomic_integral/cons/assign_neg.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/cons/copy_neg.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/cons/value_init.cc
	* testsuite/29_atomics/atomic_integral/requirements/trivial.cc: Adjust
	expected results for is_trivially_default_constructible.
	* testsuite/util/testsuite_common_types.h (has_trivial_dtor): Add
	new test generator.