[libc++] Investigate performance discrepancies with the unstable ABI

I did an experiment where I ran all of libc++'s microbenchmarks with the stable and the unstable ABI. I found some interesting functions that seemed to be a lot slower with the stable ABI than the normal ABI. Those are the top ones:

```
Benchmark Normal ABI Unstable ABI Difference % Difference
------------------------------------------------------------------------------------------------------- ------------ -------------- ------------- --------------
SetIntersection_Interlaced_Vector_tuple_262144_262144 350759492.31 1002002.85 -349757489.46 -99.71
rng::fill_n(vector)/262144 948.35 233.88 -714.47 -75.34
rng::fill(vector)/262144 900.82 235.03 -665.79 -73.91
std::fill(vector)/262144 874.51 235.02 -639.49 -73.13
rng::copy_n(vector)_(aligned)/1048576 2840.15 1038.96 -1801.19 -63.42
BM_Copy_SmallTrivialFunctor 1.24 0.49 -0.75 -60.53
BM_Copy_MemPtr 1.14 0.46 -0.68 -59.71
BM_Swap_FuncPtr_MemPtr 3.39 1.40 -1.99 -58.62
BM_Swap_MemFuncPtr_MemPtr 3.34 1.40 -1.94 -57.95
BM_Swap_FuncPtr_MemFuncPtr 3.29 1.40 -1.89 -57.38
BM_Swap_FuncPtr_SmallTrivialFunctor 3.26 1.40 -1.86 -56.97
std::fill(list)/1024 1566.32 674.74 -891.58 -56.92
std::unordered_set::insert(value)_(new_value)/8192 78.49 34.30 -44.19 -56.30
BM_Swap_MemPtr_SmallTrivialFunctor 3.18 1.40 -1.78 -55.92
BM_Swap_MemFuncPtr_SmallTrivialFunctor 3.19 1.44 -1.75 -54.87
BM_Copy_FuncPtr 1.14 0.53 -0.61 -53.57
BM_Swap_MemPtr_MemPtr 2.96 1.40 -1.55 -52.61
BM_Swap_MemFuncPtr_MemFuncPtr 2.96 1.40 -1.55 -52.54
BM_Swap_FuncPtr_FuncPtr 2.94 1.40 -1.54 -52.39
BM_Swap_SmallTrivialFunctor_SmallTrivialFunctor 2.93 1.40 -1.53 -52.13
std::vector::operator=(const&)_(cheap_elements)/32 27.29 13.44 -13.85 -50.75
rng::find_last(forward_list)_(bail_10%)/1048576 717899.62 366820.69 -351078.93 -48.90
BM_Copy_MemFuncPtr 1.14 0.60 -0.55 -47.96
rng::find_last(forward_list)_(bail_90%)/1048576 677668.97 394148.17 -283520.80 -41.84
std::vector::operator=(const&)_(cheap_elements)/1024 719.78 420.44 -299.34 -41.59
BM_ConstructAndDestroy_FuncPtr_Opaque 1.18 0.69 -0.49 -41.32
BM_PushHeap_float_Descending_1 0.70 0.41 -0.29 -40.96
std::flat_multimap::operator=(const&)_(into_cleared_Container)/8192 917.32 550.81 -366.50 -39.95
rng::move_backward(vector)/262144 23288.26 14229.98 -9058.29 -38.90
std::flat_multimap::operator=(const&)_(into_populated_Container)/8192 834.35 512.16 -322.19 -38.62
BM_RangesPopHeap_pair_1 0.70 0.43 -0.26 -37.84
BM_RangesMakeHeap_uint64_Random_262144 1.64 1.03 -0.62 -37.61
std::flat_multimap::operator=(const&)_(into_partially_populated_Container)/8192 877.22 548.19 -329.03 -37.51
BM_ConstructAndDestroy_SmallTrivialFunctor_Opaque 0.69 0.44 -0.25 -35.84
std::search(deque)_(no_match)/1024 420.60 274.49 -146.11 -34.74
BM_RangesMakeHeap_uint64_Random_16384 1.48 0.97 -0.51 -34.46
BM_MakeHeap_uint64_Random_262144 1.59 1.06 -0.54 -33.73
BM_Invoke_LargeTrivialFunctor 0.67 0.45 -0.23 -33.57
BM_Invoke_SmallTrivialFunctor 0.67 0.45 -0.23 -33.44
BM_MakeHeap_uint64_Random_16384 1.54 1.03 -0.52 -33.38
BM_RangesPushHeap_uint64_Ascending_1 0.70 0.47 -0.23 -32.98
SetIntersection_Front_Set_uint32_16384_16384 359161497.88 241585619.31 -117575878.57 -32.74
BM_RangesMakeHeap_uint64_Random_1024 1.54 1.04 -0.50 -32.43
rng::copy_n(vector)/32768 3269.99 2234.01 -1035.98 -31.68
std::unordered_map::contains(key)_(existent)/0 0.01 0.01 -0.00 -31.52
SetIntersection_Interlaced_Vector_uint64_262144_256 40066146.21 27474909.57 -12591236.64 -31.43
BM_ConstructAndDestroy_MemPtr_Opaque 0.90 0.62 -0.28 -31.27
std::flat_multimap::contains(key)_(non-existent)/0 0.65 0.45 -0.20 -31.25
SetIntersection_Front_Set_uint32_262144_16384 361161254.49 248352975.08 -112808279.40 -31.23
BM_RangesPushHeap_tuple_Random_1 0.70 0.48 -0.22 -31.03
```

Full results here: [comparison.txt](https://github.com/user-attachments/files/22752730/comparison.txt)

This should be pretty easy to reproduce locally as well. I will start filing issues as children of this one for individual investigations.