[libc++] Optimize ranges::count for __bit_iterators

```
---------------------------------------------------------------
Benchmark                                    old            new
---------------------------------------------------------------
bm_vector_bool_count/1                   1.92 ns        1.92 ns
bm_vector_bool_count/2                   1.92 ns        1.92 ns
bm_vector_bool_count/3                   1.92 ns        1.92 ns
bm_vector_bool_count/4                   1.92 ns        1.92 ns
bm_vector_bool_count/5                   1.92 ns        1.92 ns
bm_vector_bool_count/6                   1.92 ns        1.92 ns
bm_vector_bool_count/7                   1.92 ns        1.92 ns
bm_vector_bool_count/8                   1.92 ns        1.92 ns
bm_vector_bool_count/16                  1.92 ns        1.92 ns
bm_vector_bool_count/64                  2.24 ns        2.25 ns
bm_vector_bool_count/512                 3.19 ns        3.20 ns
bm_vector_bool_count/4096                14.1 ns        12.3 ns
bm_vector_bool_count/32768               84.0 ns        83.6 ns
bm_vector_bool_count/262144               664 ns         661 ns
bm_vector_bool_count/1048576             2623 ns        2628 ns
bm_vector_bool_ranges_count/1            1.07 ns        1.92 ns
bm_vector_bool_ranges_count/2            1.65 ns        1.92 ns
bm_vector_bool_ranges_count/3            2.27 ns        1.92 ns
bm_vector_bool_ranges_count/4            2.68 ns        1.92 ns
bm_vector_bool_ranges_count/5            3.33 ns        1.92 ns
bm_vector_bool_ranges_count/6            3.99 ns        1.92 ns
bm_vector_bool_ranges_count/7            4.67 ns        1.92 ns
bm_vector_bool_ranges_count/8            5.19 ns        1.92 ns
bm_vector_bool_ranges_count/16           11.1 ns        1.92 ns
bm_vector_bool_ranges_count/64           52.2 ns        2.24 ns
bm_vector_bool_ranges_count/512           452 ns        3.20 ns
bm_vector_bool_ranges_count/4096         3577 ns        12.1 ns
bm_vector_bool_ranges_count/32768       28725 ns        83.7 ns
bm_vector_bool_ranges_count/262144     229676 ns         662 ns
bm_vector_bool_ranges_count/1048576    905574 ns        2625 ns
```

Reviewed By: #libc, ldionne

Spies: arichardson, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D156956