v: implement $for comptime T.variants

[felipensp]

Fix #20176

type TestSum = int | string

fn gen[T](val T) {
    $if val is $sumtype {
        $for f in T.variants {
            dump(f)
            dump(f.typ)
            $if f.typ is $int {
                dump('is int')
            } $else $if f.typ is string {
                dump('is string')
            }
        }
    }
}

fn main() {
    a := TestSum(123)
    gen(a)
}

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