This PR adds an implementation of parallel
`std::lexicographical_compare()` based on parallel `std::mismatch()`.
The implementation is close to a one-liner:
```c++
auto __res = _Mismatch()(__policy, __first1, __last1, __first2, __last2, [&](_Ref1 __lhs, _Ref2 __rhs) {
return !__comp(__lhs, __rhs) && !__comp(__rhs, __lhs);
});
```
Included tests check that:
- Semantics of the iterator-only version is correct.
- Semantics of the predicated version is correct.
- The functions correctly SFINAE out when the first argument is not an
execution policy.
- The `noexcept` policy is followed.
- The `nodiscard` policy is followed.
- `static_assert` verifies iterators' categories.
Part of #99938.
b4d2b2f9a3