[libc++] Optimize for_each for segmented iterators
```
---------------------------------------------------
Benchmark old new
---------------------------------------------------
bm_for_each/1 3.00 ns 2.98 ns
bm_for_each/2 4.53 ns 4.57 ns
bm_for_each/3 5.82 ns 5.82 ns
bm_for_each/4 6.94 ns 6.91 ns
bm_for_each/5 7.55 ns 7.75 ns
bm_for_each/6 7.06 ns 7.45 ns
bm_for_each/7 6.69 ns 7.14 ns
bm_for_each/8 6.86 ns 4.06 ns
bm_for_each/16 11.5 ns 5.73 ns
bm_for_each/64 43.7 ns 4.06 ns
bm_for_each/512 356 ns 7.98 ns
bm_for_each/4096 2787 ns 53.6 ns
bm_for_each/32768 20836 ns 438 ns
bm_for_each/262144 195362 ns 4945 ns
bm_for_each/1048576 685482 ns 19822 ns
```
Reviewed By: ldionne, Mordante, #libc
Spies: arichardson, libcxx-commits
Differential Revision: https://reviews.llvm.org/D151274