Fix MSVC error about missing std::bit_cast

Eigen/Core

diff --git a/Eigen/Core b/Eigen/Core
index 90bcbc3..4b35b6c 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -103,6 +103,11 @@
 #include <thread>
 #endif
 
+// for std::bit_cast()
+#if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+#include <bit>
+#endif
+
 // for outputting debug info
 #ifdef EIGEN_DEBUG_ASSIGN
 #include <iostream>