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

[jloser]

Summary

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

[jloser]

Remove the now-unused concepts headers

[• Quuxplusone]

LGTM!

libcxx/include/string_view

194–195

Please check whether we can remove either/both of these headers now.

[jloser]

libcxx/include/string_view

194–195

I removed them and it should be fine since they were only added in 4be7f48960980831a9b744c96f38ddb7dc7848ce when I implemented the iterator/sentinel constructor. Nobody else is using them in this file.

libcxx/include/string_view

namespace std {
   constexpr basic_string_view<char16_t> operator "" sv( const char16_t *str, size_t len ) noexcept;
   constexpr basic_string_view<char32_t> operator "" sv( const char32_t *str, size_t len ) noexcept;
 
 }  // namespace std
 
 
 */
 
 #include <__concepts/convertible_to.h>
 #include <__concepts/same_as.h>
 #include <__config>
 #include <__debug>
 #include <__ranges/enable_borrowed_range.h>
 #include <__ranges/enable_view.h>
 #include <__string>
 #include <algorithm>
 #include <compare>
 #include <iosfwd>
public:
     {
 #if _LIBCPP_STD_VER > 11
     _LIBCPP_ASSERT(__len == 0 || __s != nullptr, "string_view::string_view(_CharT *, size_t): received nullptr");
 #endif
     }
 
 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_RANGES)
     template <contiguous_iterator _It, sized_sentinel_for<_It> _End>
-      requires (same_as<iter_value_t<_It>, _CharT> && !convertible_to<_End, size_type>)
+      requires (is_same_v<iter_value_t<_It>, _CharT> && !is_convertible_v<_End, size_type>)
     constexpr _LIBCPP_HIDE_FROM_ABI basic_string_view(_It __begin, _End __end)
        : __data(_VSTD::to_address(__begin)), __size(__end - __begin)
     {
       _LIBCPP_ASSERT((__end - __begin) >= 0, "std::string_view::string_view(iterator, sentinel) received invalid range");
     }
 #endif
 
     _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY