libstdc++: Fix visitor return type diagnostics [PR97499]

[Ville Voutilainen]

On Fri, 16 Oct 2020 at 13:02, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 16/10/20 10:26 +0300, Ville Voutilainen via Libstdc++ wrote:
> >Tested on Linux-PPC64. I haven't tested this with clang yet,
> >Jonathan, can you help with that? The previous implementation
> >indeed made an if-constexpr branch invalid for all instantiations
> >of that branch, this one doesn't - now we have just a dependent static_assert
> >which is well-formed for correct visitors and ill-formed for incorrect
> >visitors.
>
> All I checked is that <variant> can now be included using -std=c++17,
> but that works with Clang.
>
> OK for trunk, thanks.

The subject line is wrong, this is 97449, not 97499. Fixing that and pushing.