Make fixed-size Matrix and Array trivially copyable after C++20

Making them trivially copyable allows using std::memcpy() without undefined
behaviors.

Only Matrix and Array with trivially copyable DenseStorage are marked as
trivially copyable with an additional type trait.

As described in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0848r3.html
it requires extremely verbose SFINAE to make the special member functions of
fixed-size Matrix and Array trivial, unless C++20 concepts are available to
simplify the selection of trivial special member functions given template
parameters. Therefore only make this feature available to compilers that support
C++20 P0848R3.

Fix #1855.

Eigen/src/Core/util/ForwardDeclarations.h

@@ -42,6 +42,11 @@ template<typename T> struct evaluator_traits;
 
 template< typename T> struct evaluator;
 
+template<typename T> struct has_trivially_copyable_storage
+{
+  static const bool value = false;
+};
+
 } // end namespace internal
 
 template<typename T> struct NumTraits;