cgen: fix reference variable str() method call (fix #19430)

[yuyi98]

This PR fix reference variable str() method call (fix #19430).

• Fix reference variable str() method call.
• Add test.

@[heap]
struct Foo {
	bar string = 'bar'
	baz string = 'baz'
}

fn (foo Foo) str() string {
	return 'bar: ${foo.bar}, baz: ${foo.baz}'
}

fn main() {
	mut many_foos := []&Foo{len: 3, init: &Foo{}}
	println(many_foos.map(it.str()).join('\n'))
	println(many_foos)
	assert many_foos.map(it.str()) == ['&bar: bar, baz: baz', '&bar: bar, baz: baz',
		'&bar: bar, baz: baz']
}

PS D:\Test\v\tt1> v run .
&bar: bar, baz: baz
&bar: bar, baz: baz
&bar: bar, baz: baz
[&bar: bar, baz: baz, &bar: bar, baz: baz, &bar: bar, baz: baz]

[Delta456]

I believe the OP links to the incorrect issue

[yuyi98]

@Delta456 Thanks! My mistake.

[spytheman]

Good work.