cgen: fix code generated for generic option fixed array return type

[felipensp]

Fix #20465

fn example[T]() ?T {
	return T{}
}

fn test_main() {
	dump(example[[1]int]())

	a := example[[1]int]()
	assert a? == [0]!
}

[spytheman]

Excellent work.

[spytheman]

A very similar test, also produces a cgen error, not sure if it is related/easy to fix:

fn example[T]() !T {
	return T{}
}

fn test_main() {
	dump(example[[1]int]()!)

	a := example[[1]int]()!
	assert a == [0]!
}