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

[Jonathan Wakely]

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.