Make bit manipulation functions constexpr on MSVC 19.25+. Closes #109.

test/bit_popcount_test_cx.cpp

@@ -4,6 +4,10 @@
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
+#if defined(_MSC_VER) && _MSC_VER / 10 == 191
+# pragma warning(disable: 4307) // '*': integral constant overflow
+#endif
+
 #include <boost/config.hpp>
 #include <boost/config/pragma_message.hpp>