UCS_NONBMP_W2_FLAG_BIT
#define UCS_NONBMP_W2_FLAG (1u << UCS_NONBMP_W2_FLAG_BIT)
static_assert(UCS_NONBMP_BMP_BITS <= UCS_NONBMP_W2_FLAG_BIT,
static_assert(UCS_NONBMP_W2_FLAG_BIT < UCS_CP_SHIFT,