atomic_bool missing

Seems libc++ <atomic> is missing 'typedef atomic<bool> atomic_bool;'. This
requirement is easy to overlook in the text of the standard.

    [§29.5p7] There shall be named types corresponding to the integral
    specializations of atomic, as specified in Table 145, and a named type
                                                          ^^^^^^^^^^^^^^^^
    atomic_bool corresponding to the specified atomic<bool>. Each named type is
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    a either typedef to the corresponding specialization or a base class of the
    corresponding specialization. If it is a base class, it shall support the
    same member functions as the corresponding specialization.

         Table 145 — atomic integral typedefs

        Named type Integral argument type
        --------------- ----------------------
        atomic_char char
        atomic_schar signed char
        atomic_uchar unsigned char
        atomic_short short
        atomic_ushort unsigned short
        atomic_int int
        atomic_uint unsigned int
        atomic_long long
        atomic_ulong unsigned long
        atomic_llong long long
        atomic_ullong unsigned long long
        atomic_char16_t char16_t
        atomic_char32_t char32_t
        atomic_wchar_t wchar_t

— Gordon