[libcxx] Optimize std::generate for segmented iterators (#163006)

Part of #102817.

This patch attempts to optimize the performance of `std::generate` for
segmented iterators. Below are the benchmark numbers from
`libcxx\test\benchmarks\algorithms\modifying\generate.bench.cpp`. Test
cases that use segmented iterators have also been added.

- before

```
std::generate(deque<int>)/32           194 ns          193 ns      3733333
std::generate(deque<int>)/50           276 ns          276 ns      2488889
std::generate(deque<int>)/1024        5096 ns         5022 ns       112000
std::generate(deque<int>)/8192       40806 ns        40806 ns        17231
```

- after

```
std::generate(deque<int>)/32           106 ns          105 ns      6400000
std::generate(deque<int>)/50           139 ns          138 ns      4977778
std::generate(deque<int>)/1024        2713 ns         2699 ns       248889
std::generate(deque<int>)/8192       18983 ns        19252 ns        37333
```

---------

Co-authored-by: A. Jiang <de34@live.cn>

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