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.

unsupported/Eigen/CXX11/ThreadPool

@@ -39,9 +39,7 @@
 #include <condition_variable>
 #include <deque>
 #include <mutex>
-#include <thread>
 #include <functional>
-#include <memory>
 #include <utility>
 
 // There are non-parenthesized calls to "max" in the  <unordered_map> header,
@@ -53,6 +51,15 @@
 #endif
 #include <unordered_map>
 
+// C++20 libstdc++'s headers also calls "max" and "min" indirectly.
+#if EIGEN_COMP_HAS_P0848R3
+#ifdef min
+#undef min
+#endif
+#endif
+#include <thread>
+#include <memory>
+
 #include "src/util/CXX11Meta.h"
 #include "src/util/MaxSizeVector.h"