add cross platform counted_by defines in test case

Author: hnrklssn

test/Interop/Cxx/stdlib/Inputs/std-span.h

@@ -6,6 +6,14 @@
 #include <string>
 #include <vector>
 
+#ifndef __counted_by // cstddef already imports ptrcheck.h on apple platforms
+#if defined(__has_feature) && __has_feature(bounds_safety_attributes)
+  #define __counted_by(x) __attribute__((__counted_by__(x)))
+#else
+  #define __counted_by(x)
+#endif
+#endif
+
 using ConstSpanOfInt = std::span<const int>;
 using SpanOfInt = std::span<int>;
 using ConstSpanOfString = std::span<const std::string>;