[committed] libstdc++: Add deduction guide for std::ranges::join_view [LWG 3474]

[Tim Song]

I thought LWG approved the other option in the PR (changing views::join to
not use CTAD)?

On Mon, Aug 24, 2020 at 10:22 AM Jonathan Wakely via Gcc-patches <
gcc-patches@gcc.gnu.org> wrote:

> This implements the proposed resolution for LWG 3474.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (join_view): Add deduction guide (LWG 3474).
>         * testsuite/std/ranges/adaptors/join_lwg3474.cc: New test.
>
> Tested powerpc64le-linux. Committed to trunk.
>
>