@:keep present while the class is DCEd

When using `-dce full`, some classes seem to received a free `@:keep` meta automatically, while that forced `@:keep` does not have the expected effect: the unused class fields are DCEd.

This does not happen when using default `-dce std` arg

This leads to inconcistency when trying to check for the presence of `@:keep` through RTTI for example: it's appears to be present, but it "didn't work".

HAXE
```haxe
@:rtti class Foo {
public static function main() {
var f = new Foo();
trace( Reflect.hasField(f, "bar") ); // false
trace( haxe.rtti.Rtti.getRtti(Foo).meta ); // @:keep appears here
}

public function new() { }

public function bar() { } // this function is DCEd
}
```

HXML
```hxml
-main Foo
-dce full
```

Reproduce the Haxe example with the shown HXML entry point, comparing -dce full with the default -dce std. Inspect the interaction between Reflect.hasField, haxe.rtti.Rtti.getRtti(Foo), @:keep, and dead-code elimination; done means kept fields remain available and RTTI no longer reports misleading metadata.

Written by the indexing model from the issue text.