Decrease the internal count of the std::experimental::latch object by n.
void count_down(ptrdiff_t n);
n>=0 and n<=count,
where count is
the internal counter value of the latch.
Reduces the internal counter value of *this by n.
If the counter is reduced to zero, the latch becomes ready
and all waiting threads block in std::experimental::latch::wait or std::experimental::latch::count_down_and_wait are
unblocked.
If the counter is reduced to zero, is_ready()==true.
#include <experimental/latch>