libstdc++: Fix constexpr constructor for atomic<shared_ptr<T>>

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_atomic.h (_Sp_atomic::_Atomic_count):
	Add constexpr.
	(_Sp_atomic::_M_ptr): Add default member-initializer.
	* testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc:
	Check constant initialization.
	* testsuite/20_util/weak_ptr/atomic_weak_ptr.cc: Likewise.
3 files changed