[libc++][NFC] Mark LWG3573 as complete

Mark LWG3573 as complete. It involves a change in wording around when
`basic_string_view`'s constructor for iterator/sentinel can throw. The
current implementation is not marked conditionally `noexcept`, so there
is nothing to do here. Add a test that binds this behavior to verify the
constructor is not marked `noexcept(true)` when `end - begin` throws.

Reviewed By: ldionne, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D111925