When we ship LLVM 16, <ranges> won't be considered experimental anymore.
We might as well do this sooner rather than later.
Differential Revision: https://reviews.llvm.org/D132151
All supported compilers that support C++20 now support concepts. So, remove
`_LIB_LIBCPP_HAS_NO_CONCEPTS` in favor of `_LIBCPP_STD_VER > 17`. Similarly in
the tests, remove `// UNSUPPORTED: libcpp-no-concepts`.
Differential Revision: https://reviews.llvm.org/D121528
Change the tests to use the base friend function instead of members.
Also changed some types to have a base friends instead of members.
Reviewed By: #libc, ldionne, Quuxplusone
Differential Revision: https://reviews.llvm.org/D120742
Louis Dionne