[libcxx] Optimize `std::generate_n` for segmented iterators (#164266)
Part of #102817.
This is a natural follow-up to #163006. We are forwarding
`std::generate_n` to `std::__for_each_n` (`std::for_each_n` needs
c++17), resulting in improved performance for segmented iterators.
before:
```
std::generate_n(deque<int>)/32 17.5 ns 17.3 ns 40727273
std::generate_n(deque<int>)/50 25.7 ns 25.5 ns 26352941
std::generate_n(deque<int>)/1024 490 ns 487 ns 1445161
std::generate_n(deque<int>)/8192 3908 ns 3924 ns 179200
```
after:
```
std::generate_n(deque<int>)/32 11.1 ns 11.0 ns 64000000
std::generate_n(deque<int>)/50 16.1 ns 16.0 ns 44800000
std::generate_n(deque<int>)/1024 291 ns 292 ns 2357895
std::generate_n(deque<int>)/8192 2269 ns 2250 ns 298667
```
GitOrigin-RevId: c06ae43e26aa5cd472d0b25d5904c44d20e84067
3 files changed
