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;

Eigen/src/Core/util/Macros.h

@@ -1084,6 +1084,16 @@ namespace Eigen {
     }
 #endif
 
+/**
+ * \internal
+ * \brief Macro for conditionally trivial special member functions for supporting trivially copyable types.
+ * This feature is officially known as C++20's P0848R3 and is enabled on supported compilers.
+ */
+#if (EIGEN_COMP_CXXVER >= 20) && (EIGEN_COMP_GNUC_STRICT || EIGEN_COMP_MSVC_STRICT)
+#define EIGEN_COMP_HAS_P0848R3 1
+#else
+#define EIGEN_COMP_HAS_P0848R3 0
+#endif
 
 /**
  * \internal