ifdef removed from MapBase and warning disabled

Eigen/src/Core/MapBase.h

@@ -180,12 +180,10 @@ template<typename Derived> class MapBase
                   && cols >= 0 && (ColsAtCompileTime == Dynamic || ColsAtCompileTime == cols)));
     }
 
-#ifndef _MSC_VER
     Derived& operator=(const MapBase& other)
     {
       return Base::operator=(other);
     }
-#endif
 
     using Base::operator=;
     using Base::operator*=;