"output_value: functional value" when using hl.Ref<String> in hl/c

Minimal repro:
```
@:hlNative("test")
class NativeTest
{
public static function refFunc(value: hl.Ref) : Void {}
}

class Main {
static function main() {
var test: String = "";
NativeTest.refFunc( test );
}
}
```

This error is produced by the haxe compiler during hl/c generation (`-hl out.c`). It works as expected when using hl bytecode.

hl.Ref\ and other types work fine, only String seems to be affected.