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

[Ville Voutilainen]

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.

2020-10-16  Ville Voutilainen  <ville.voutilainen@gmail.com>

    PR libstdc++/97449
    * include/std/variant
    (__gen_vtable_impl<>::_S_apply_single_alt):
    Diagnose visitor return type mismatches here..
    (__gen_vtable_impl</*base case*/>::_S_apply):
    ..not here.
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