[libc++] LWG 3857: allow `string_view` conversion when only traits vary

The `basic_string_view` constructor accepting a contiguous range rejects
converting between `basic_string_view` even when only the trait types vary.
This prevents conversions for converting from `basic_string_view<C, T1>` and
`basic_string<C, T1, A>` to `basic_string_view<C, T2>`.  Recently, this
constructor was made `explicit`, so there's no reason to really forbid this
conversion anymore.

Relax the restriction that the trait types need to match in this constructor.

Differential Revision: https://reviews.llvm.org/D143972