libstdc++: Use type_identity_t for non-deducible std::atomic_xxx args

libstdc++: Use type_identity_t for non-deducible std::atomic_xxx args

This is LWG 3220 which is about to become Tentatively Ready.

libstdc++-v3/ChangeLog:

	* include/std/atomic (__atomic_val_t): Use __type_identity_t
	instead of atomic<T>::value_type, as per LWG 3220.
	* testsuite/29_atomics/atomic/lwg3220.cc: New test.

(cherry picked from commit 30cc1b65e4efa1a2c57fec5574fcae7a446b822f)