[libc++] flat_set uses 2x as many comparisons as it should

```c++
#include
#include

int Count = 0;
struct Comp { bool operator()(int x, int y) const { ++Count; return x < y; } };

int main() {
std::flat_set x = {0,0,0,0,0,0,0,0,0,0};
std::cout << Count;
}
```

This program uses 27 comparisons. It can be done in 18. The problem is that `flat_set::__key_equiv` always calls the comparator twice instead of once. I will post a patch for this.

Start by locating libc++'s flat_set implementation and the flat_set::__key_equiv helper named in the issue. Run the supplied reproducer to confirm the 27 comparisons, then verify that the completed change reduces this case to 18 without changing flat_set behavior. The issue does not name a source file or test.

Written by the indexing model from the issue text.