Implement P0558R2 changes to std::atomic

Implement P0558R2 changes to std::atomic

The restrictions forbidding arithmetic on atomic pointer types are only
enabled for C++17 and later, retaining the GNU extension for older
standards. The new nested typedefs and changes to prevent scalar
parameters participating in template argument deduction are enabled
unconditionally.

	PR libstdc++/69769
	PR libstdc++/85886
	* include/bits/atomic_base.h (__atomic_base::value_type)
	(__atomic_base::difference_type): Add new typedefs.
	* include/std/atomic (atomic<bool>::value_type, atomic<T>::value_type)
	(atomic<T*>::value_type, atomic<T*>::difference_type): Likewise.
	(atomic<T*>::operator++, atomic<T*>::operator--)
	(atomic<T*>::operator+=, atomic<T*>::operator-=)
	(atomic<T*>::fetch_add, atomic<T*>::fetch_sub): Add static assertion
	to enforce C++17 requirement on pointer arithmetic.
	(__atomic_val_t, __atomic_diff_t): New alias templates.
	(atomic_init, atomic_store_explicit, atomic_exchange_explicit)
	(atomic_compare_exchange_weak_explicit)
	(atomic_compare_exchange_strong_explicit, atomic_store)
	(atomic_exchange, atomic_compare_exchange_weak)
	(atomic_compare_exchange_strong): Use __atomic_val_t to make
	scalar parameters be non-deduced contexts.
	(atomic_fetch_add_explicit, atomic_fetch_sub_explicit)
	(atomic_fetch_add, atomic_fetch_sub): Change first parameter to be
	atomic instead of __atomic_base, and use __atomic_diff_t for scalar
	parameters.
	(atomic_fetch_and_explicit, atomic_fetch_or_explicit)
	(atomic_fetch_xor_explicit, atomic_fetch_and, atomic_fetch_or)
	(atomic_fetch_xor): Use __atomic_val_t for scalar parameters.
	(atomic_fetch_add_explicit, atomic_fetch_sub_explicit)
	(atomic_fetch_add, atomic_fetch_sub): Remove overloads for atomic
	address types.
	* testsuite/29_atomics/atomic/60695.cc: Adjust dg-error lineno.
	* testsuite/29_atomics/atomic/69769.cc: New test.
	* testsuite/29_atomics/atomic/nonmembers.cc: New test.
	* testsuite/29_atomics/atomic/operators/pointer_partial_void.cc:
	Disable test for C++17 and later.
	* testsuite/29_atomics/atomic/requirements/typedefs.cc: New test.
	* testsuite/29_atomics/atomic_integral/nonmembers.cc: New test.
	* testsuite/29_atomics/atomic_integral/requirements/typedefs.cc: New
	test.

From-SVN: r260676