We should enforce `/Zc:__cplusplus` by default in our CMake basic target

[saki7]

Currently, our detection of the forward declaration of std::hash is broken because MSVC sets __cplusplus to 199711L by default. This is a well-known long-standing burden in MSVC which I think should be fixed in our _iris_cxx_common target.

iris/include/iris/hash_fwd.hpp

Lines 8 to 16 in 325aec9

	#if __cplusplus <= 202302L || \
	(defined(_MSVC_STL_UPDATE) && _MSVC_STL_UPDATE < 202504L) || \
	(defined(__GLIBCXX__) && __GLIBCXX__ < 20241201) || \
	(defined(_LIBCPP_STD_VER) && _LIBCPP_STD_VER < 26)
	# include <typeindex> // pre-C++26
	#else
	# include <utility> // C++26 and later
	#endif