[CSBindings] Adjust hasConversions to handle Void has having not conversions

Tuples in general do have conversions but an empty tuple or `Void`
doesn't, which means that if a type variable has a subtype binding
to `Void` it should be safe to prioritize.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -1269,7 +1269,8 @@ static bool hasConversions(Type type) {
   }
 
   return !(type->is<StructType>() || type->is<EnumType>() ||
-           type->is<BuiltinType>() || type->is<ArchetypeType>());
+           type->is<BuiltinType>() || type->is<ArchetypeType>() ||
+           type->isVoid());
 }
 
 bool BindingSet::favoredOverDisjunction(Constraint *disjunction) const {