cgen: fix interface eq method with option and ref(fix #19441)

[shove70]

1. Fixed Option field causes C error when struct embeds multiple other structs #19441
2. Add tests.

pub interface NodeInterface {
mut:
	node_name string
}

pub interface Element {
	NodeInterface
mut:
	tag_name string
}

pub struct Node {
pub mut:
	node_name string
}

pub struct HTMLElement {
	Node
pub mut:
	tag_name string
}

pub struct HTMLInputElement {
	// removing either one of these embedded structs lets the code compile
	HTMLElement
	PopoverInvokerElement
pub mut:
	value string
}

pub struct PopoverInvokerElement {
pub mut:
	// changing this field to `&Element` lets the code compile
	popover_target_element ?&Element
	popover_target_action  string
}

fn main() {
	mut arr := []&NodeInterface{}
	arr << &HTMLElement{}
	arr << &HTMLElement{}

	// removing this comparison lets the code compile
	println(arr[0] == arr[1])
}

outputs:

true

[shove70]

By the way, the problem of repeatedly generating the same eq_method is solved.

interface Any{}

struct Foo {
	field &Any
}

fn main() {
	foo1 := &Any(Foo {
		&Any(1)
	})
	foo2 := &Any(Foo {
		&Any(1)
	})
	
	println(foo1 == foo2)
}

outputs:

Running code...
Can't run code. The server returned an error:
/tmp/v_60000/code.377993553552140351.tmp.c:2181: error: redefinition of 'main__Any_interface_eq'
builder error: 
==================
C error. This should never happen.

[shove70]

@spytheman spy,

This needs to be merged, for the green CI:
vlang/ved#173

thank you! :)

[spytheman]

This needs to be merged, for the green CI: vlang/ved#173

Done. Thanks for fixing it.