[libcxx] Unwrap iterators in __find_segment (#161274)
The segmented iterator optimized implementation of find now unwraps
iterators when processing each segments.
As a result, it is able to take better advantage to some find
specializations: calling memchr/wmemchr for vector<vector<{char,int}>>
```
Benchmark Baseline Candidate Difference % Difference
-------------------------------------------------------------- ---------- ----------- ------------ --------------
rng::find(join_view(deque<deque<int>>))_(process_all)/1024 71.13 61.19 -9.94 -13.97
rng::find(join_view(deque<deque<int>>))_(process_all)/32768 2359.19 2237.02 -122.17 -5.18
rng::find(join_view(deque<deque<int>>))_(process_all)/50 16.88 17.59 0.71 4.20
rng::find(join_view(deque<deque<int>>))_(process_all)/8 15.59 16.10 0.51 3.27
rng::find(join_view(deque<deque<int>>))_(process_all)/8192 647.01 532.75 -114.26 -17.66
rng::find(join_view(list<vector<int>>))_(process_all)/1024 689.76 680.74 -9.02 -1.31
rng::find(join_view(list<vector<int>>))_(process_all)/32768 22284.95 21500.26 -784.69 -3.52
rng::find(join_view(list<vector<int>>))_(process_all)/50 32.77 32.12 -0.65 -1.98
rng::find(join_view(list<vector<int>>))_(process_all)/8 6.11 5.92 -0.19 -3.11
rng::find(join_view(list<vector<int>>))_(process_all)/8192 5527.88 5373.43 -154.45 -2.79
rng::find(join_view(vector<list<int>>))_(process_all)/1024 1305.59 1264.04 -41.55 -3.18
rng::find(join_view(vector<list<int>>))_(process_all)/32768 42840.88 43322.64 481.76 1.12
rng::find(join_view(vector<list<int>>))_(process_all)/50 57.52 62.35 4.82 8.38
rng::find(join_view(vector<list<int>>))_(process_all)/8 6.06 5.98 -0.07 -1.18
rng::find(join_view(vector<list<int>>))_(process_all)/8192 20700.53 21431.66 731.12 3.53
rng::find(join_view(vector<vector<char>>))_(process_all)/1024 310.64 18.34 -292.30 -94.09
rng::find(join_view(vector<vector<char>>))_(process_all)/32768 9424.96 531.99 -8892.97 -94.36
rng::find(join_view(vector<vector<char>>))_(process_all)/50 18.58 3.25 -15.32 -82.49
rng::find(join_view(vector<vector<char>>))_(process_all)/8 4.81 2.98 -1.84 -38.13
rng::find(join_view(vector<vector<char>>))_(process_all)/8192 2437.50 126.88 -2310.62 -94.79
rng::find(join_view(vector<vector<int>>))_(process_all)/1024 297.10 41.70 -255.39 -85.96
rng::find(join_view(vector<vector<int>>))_(process_all)/32768 9662.42 1822.05 -7840.36 -81.14
rng::find(join_view(vector<vector<int>>))_(process_all)/50 22.29 5.10 -17.19 -77.11
rng::find(join_view(vector<vector<int>>))_(process_all)/8 3.73 3.13 -0.60 -16.05
rng::find(join_view(vector<vector<int>>))_(process_all)/8192 2399.68 356.10 -2043.58 -85.16
```
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