libstdc++: Deprecate std::atomic_xxx overloads for std::shared_ptr
These overloads are deprecated in C++20 (and likely to be removed for
C++26). The std::atomic<std::shared_ptr<T>> specialization should be
preferred in new code.
libstdc++-v3/ChangeLog:
* include/bits/shared_ptr_atomic.h (atomic_is_lock_free)
(atomic_load_explicit, atomic_load, atomic_store_explicit)
(atomic_store, atomic_exchange_explicit, atomic_exchange)
(atomic_compare_exchange_strong, atomic_compare_exchange_weak)
(atomic_compare_exchange_strong_explicit)
(atomic_compare_exchange_weak_explicit): Add deprecated
attribute for C++20 and later.
* testsuite/20_util/shared_ptr/atomic/1.cc: Suppress deprecated
warnings.
* testsuite/20_util/shared_ptr/atomic/2.cc: Likewise.
* testsuite/20_util/shared_ptr/atomic/3.cc: Likewise.
* testsuite/29_atomics/atomic/lwg3220.cc: Likewise.