Regression after 1.62: `variant<recursive_wrapper<X>, ...>` is not constructible from `X` when `X` is a variant

Regressed in https://github.com/boostorg/variant/commit/927280570a553b43846e74ddd6b3802d5a1a5f1c
```cpp
#include

struct Nil {};

typedef boost::variant<
Nil
, double
>
Atom;

typedef boost::variant<
Nil
#if 1
, boost::recursive_wrapper
#else
, Atom
#endif
>
Base;

int main()
{
Atom atom;
Base base = atom;
}
```

Removing these specializations fixes the compilation https://github.com/boostorg/variant/blob/d2fdf2384b146b0c764077f5539f5f6ed7c40f20/include/boost/variant/recursive_wrapper_fwd.hpp#L60-L67