fixes #20699; object variants with void fields crash with ORC

[ringabout]

fixes #20699

Different from refc, ORC initializes object variants. If an object variant has void fields, it will get a wrong initial value. In fact, it shouldn't be assigned with a default value since the void field is eliminated from types.

[deech]

Instead of omitting the struct initialization in the presence of a void why not have a special empty Void struct in the type declaration? That way it is more explicit. I think it's undefined behavior technically but compilers seem to support it.

typedef struct Void Void;
struct Void {};

typedef struct SomeObject SomeObject;
struct SomeObject {
  void* payload;
};

typedef struct Variant Variant;
struct Variant {
  char kind;
  union{
    Void _kind_1;  
    struct {
      SomeObject a;
    } _kind_2;
  };
};

static const Variant V = {0, {._kind_1 = {}}};

[bung87]

it's a large refactoring rather than fix one specific issue. nim-lang/RFCs#508