cgen: fix sumtype with embedded struct of option field (fix #22984)

[yuyi98]

This PR fix sumtype with embedded struct of option field (fix #22984).

• Fix sumtype with embedded struct of option field.
• Add test.

struct Value {
	x int
}

struct BValue {
	v ?Value
}

struct Word {
	BValue
}

struct Long {
	BValue
}

struct Variadic {
	BValue
}

type Param = Word | Long | Variadic

fn main() {
	a := [Param(Word{}), Long{}, Variadic{}]
	dump(a)
	f := a[0]
	v := f.v
	dump(v)
	assert v == none
}

PS D:\Test\v\tt1> v run .
[.\\tt1.v:25] a: [Param(Word{
    BValue: BValue{
        v: Option(none)
    }
}), Param(Long{
    BValue: BValue{
        v: Option(none)
    }
}), Param(Variadic{
    BValue: BValue{
        v: Option(none)
    }
})]
[.\\tt1.v:28] v: Option(none)

_{Huly®: V_0.6-21437}

[spytheman]

Excellent work.