[libc++] Fix `std::pair`'s pair-like constructor's incorrect assumption (#66585)

The helper function `__pair_like_explicit_wknd` is only SFINAE-ed with
`tuple_size<remove_cvref_t<_PairLike>>::value == 2`, but its function
body assumes `std::get` being valid.

Fixes #65620

(cherry picked from commit 054f9c55)