Use escape hook for removal of <ciso646>

<ciso646> has been removed in C++20, which breaks the build against MSVC. Fortunately, there is an escape hatch that we can use to silence the deprecation warning and buy us some time until we settle on a better solution

.upstream-tests/test/support/msvc_stdlib_force_include.h

@@ -78,6 +78,7 @@ const AssertionDialogAvoider assertion_dialog_avoider{};
     #define _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS
 #endif // _LIBCXX_IN_DEVCRT
 
+#define _SILENCE_CXX20_CISO646_REMOVED_WARNING
 #include <ciso646>
 
 #if _HAS_CXX20