rvalue variant with lvalue references does not compile

As far as I can tell, the following is a legitimate code which attempts to a temporary variant to keep references. But it fails to compile:

```.cpp
struct Changer : boost::static_visitor {
void operator()(int& i) const {
i += 10;
}
void operator()(double& d) const {
d += 20;
}
};

auto getVar(int choose, int& i, double& d){
using V = boost::variant;
return choose == 0 ? V(i) : V(d);
}

void foo() {
int i = 0;
double d = 0;
Changer c;
auto v1 = getVar(0,i,d);
boost::apply_visitor(c, v1); // OK
assert(i == 10);
assert(d == 0.);
boost::apply_visitor(c, getVar(1,i,d)); // ERROR
assert(i == 10);
assert(d == 20.);
}
```

```
error: no matching function for call to object of type 'Changer'
```

See https://godbolt.org/z/FVVA_t for the error.

It seems `apply_visitor` is making the assumption that if a variant is rvalue, so should be the elements. But (unlike std::variant) boost::variant supports references as elements, so the two are decoupled.

This makes variants impossible to use as proxy objects.