[cpp] Add missing override specifiers

[tobil4sk]

Together with HaxeFoundation/hxcpp#1319, this fixes warnings about missing overrides that show up when compiling a hxcpp program.

These were already present in a few places, and it is fine to add them because the minimum c++ version for haxe 5 is already c++11. It is good practice to have these to help catch issues like #11666, where a method is overloaded instead of overridden.

I had to modify the test for #9516, because adding these revealed a possible bug. Despite override in haxe code, Baz.fun was never actually overriding Bar.fun, because Foo is marked as @:nativeGen. This happens even with haxe 4.3.7, but is only revealed when these override specifiers are added.

[tobil4sk]

@Aidan63 does this seem good to you? You mentioned you ran into some weird issues when you last tried this.

[Aidan63]

I think this looks good. I'm guessing I was getting tripped up by marking all toString functions as overridden, even if they had arguments, because I don't remember doing that.

[tobil4sk]

Yeah the toString situation is a bit finicky. The genCppia code should probably check for arguments too because currently it just uses the name: #12375, but that's a separate issue.

I guess I should also check that a virtual toString in a @:nativeGen class doesn't automatically become override because that doesn't actually inherit from hx::Object.

Btw, in that test case that I had to modify:

@:nativeGen
@:structAccess
private class Foo {
  // ...
  public function fun() return /*...*/;
}

The problem is that Foo.fun is not overridden in any haxe code, so it assumes it is non virtual. Do you know if there is any way to tell haxe that Foo might be extended in external code? I tried making it public, but that doesn't help either, I also found @:nonVirtual but that is the opposite.

[Aidan63]

For native gen classes you might just have to assume that every function is virtual unless otherwise specified (final, nonVirtual), since thats the haxe semantics. I don't think there's any work around for that.

[tobil4sk]

I think haxe decides somewhere else to make it non-virtual, because otherwise virtual would get added automatically. From what I can tell the behaviour is the same for @:nativeGen and normal classes, where it assumes it's not virtual unless it is overridden in the haxe program.

I'm testing this with haxe 4 and nightly:

@:nativeGen // or commented out
class Parent {
  public function fun() return 42;
}

@:nativeGen
class Child extends Parent {
  override function fun() return 10;
}

If I comment out Child or Child.fun, then haxe automatically makes Parent.fun non virtual.