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

[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
well.

This observation came up while working on
https://reviews.llvm.org/D113161

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