libcxx: update to LLVM 18

release/18.x branch, commit 78b99c73ee4b96fe9ce0e294d4632326afb2db42

This adds the flag `-D_LIBCPP_HARDENING_MODE` which is determined based
on the Zig optimization mode.

This commit also fixes libunwind, libcxx, and libcxxabi to properly
report sub compilation errors.

lib/libcxx/include/__algorithm/ranges_set_difference.h

@@ -30,6 +30,9 @@
 #  pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 #if _LIBCPP_STD_VER >= 20
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -100,4 +103,7 @@ inline constexpr auto set_difference = __set_difference::__fn{};
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER >= 20
+
+_LIBCPP_POP_MACROS
+
 #endif // _LIBCPP___ALGORITHM_RANGES_SET_DIFFERENCE_H