Bug 17015 – support final switch in std.variant

As e.g. in https://github.com/s-ludwig/taggedalgebraic: final switch (taggedAny.kind) { case Tagged.Kind.i: // It's "int i" break; case Tagged.Kind.str: // It's "string str" break; case Tagged.Kind.foo: // It's "Foo foo" break; } this has the advantage over visit that the compiler can enforce that all types are covered.

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