[test] Define _LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST in msvc_stdlib_force_include.hpp

...so the tests under test/std/utilities/any continue to
compile with MSVC's standard library.

While we're here, let's test >C++17 features when _HAS_CXX20.

llvm-svn: 351991

libcxx/test/support/msvc_stdlib_force_include.hpp

@@ -78,10 +78,14 @@ const AssertionDialogAvoider assertion_dialog_avoider{};
 
 #include <ciso646>
 
-#if _HAS_CXX17
+#if _HAS_CXX20
+    #define TEST_STD_VER 99
+#elif _HAS_CXX17
     #define TEST_STD_VER 17
-#else // _HAS_CXX17
+#else // !(_HAS_CXX20 || _HAS_CXX17)
     #define TEST_STD_VER 14
-#endif // _HAS_CXX17
+#endif
+
+#define _LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST
 
 #endif // SUPPORT_MSVC_STDLIB_FORCE_INCLUDE_HPP