cgen: fix interface casting in anon fn (fix #23530)

[yuyi98]

This PR fix interface casting in anon fn (fix #23530).

• Fix interface casting in anon fn.
• Add test.

@[heap]
struct Foo {
mut:
	val int
}

@[heap]
struct Bar {
mut:
	val int
}

interface FooBar {
mut:
	val int
}

fn main() {
	mut fbs := []&FooBar{}
	fbs << &Foo{1}
	do_something := fn [mut fbs] () {
		_ := fbs.last() as Foo // this line works outside of anon fn
	}
	do_something()
        assert true
}

PS D:\Test\v\tt1> v run .

[huly-for-github]

Connected to Huly®: V_0.6-21961

[spytheman]

Excellent work.