Stephan Tolksdorf: fixes the issue in the <atomic> header and adds corresponding tests. I've used macros to fall back to a user-provided default constructor if _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS (though I suspect that there won't be many users defining that macro).

The tests use placement new to check that atomic values get properly zero-initialized. I had to modify the atomic_is_lock_free test, because default initialization of an object of const type 'const A' (aka 'const atomic<int>') requires a user-provided default constructor.