[libc++] Fix constraints for string_view's iterator/sentinel constructor

The `string_view` constructor taking an iterator/sentinel uses concepts
instead of type traits like the Standard states. Using `same_as` instead
of `is_same_v` should be harmless. Prefer `std::is_same_v` instead which is
cheaper to compile. Replace `convertible_to` with `is_convertible_v` as

This observation came up while working on

Differential Revision: https://reviews.llvm.org/D114561

GitOrigin-RevId: 68e7e76a9be022427e572101cde4e00203c9af46
1 file changed