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/MatrixBase.h

@@ -140,6 +140,11 @@ template<typename Derived> class MatrixBase
     EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
     Derived& operator=(const MatrixBase& other);
 
+#if EIGEN_COMP_HAS_P0848R3
+    EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE MatrixBase& operator=(
+        const MatrixBase& other) requires internal::has_trivially_copyable_storage<Derived>::value = default;
+#endif
+
     // We cannot inherit here via Base::operator= since it is causing
     // trouble with MSVC.