checker: remove unnecessary struct ref field initialization checks and notifications at map initializing(fix #20245)

[shove70]

1. Close structs containing references cannot be used in maps #20245
2. Remove unnecessary notifications
3. Handle with the implicit creation of map instances later

[spytheman]

No, please make a fix for the notice for mut foo := map[string]MyStruct{} only, without turning the notice into an error (yet).

The time for doing this is not now, given that we are still finding edge cases, where the message should not be shown at all.

[spytheman]

If we do this now (turn the notice into an error), all it would do is force people to use unsafe {} a lot more, even for cases, where it is not needed, like here:

struct MyStruct {
        a &int
        b int
}

fn main() {
        mut foo := unsafe { map[string]MyStruct{} }
        println(foo)
}

As you can see, the map is empty. No instances were initialized, and thus no message should be shown.

[shove70]

OK, I see. My main concern is that map creates instances implicitly:

struct MyStruct {
	a &int
	b int
}

fn main() {
	mut foo := map[string]MyStruct{}
	println((foo['key'].a))
}

outputs:

&nil

I'm going to follow your advice, keep the other "notifications" unchanged, and cancel the "notifications" here,
Then I'll see if map needs to be handled when it's implicitly created

[spytheman]

My main concern is that map creates instances implicitly

Yes, that is true. I think we need another checker notice/error for that, nudging the user to use m[k] or { some_default } for those cases, where the map value type has references in it.

[shove70]

Yes, that is true. I think we need another checker notice/error for that, nudging the user to use m[k] or { some_default } for those cases, where the map value type has references in it.

Ok, I'll do it. :)