Add NODISCARD macro

This is used to mark functions for which the result should not be
ignored, and can be checked by some compilers.

Author: romainbrenguier

src/util/nodiscard.h

@@ -0,0 +1,23 @@
+/*******************************************************************\
+
+Module: Util
+
+Author: Diffblue Ltd.
+
+\*******************************************************************/
+
+#ifndef CPROVER_UTIL_NODISCARD_H
+#define CPROVER_UTIL_NODISCARD_H
+
+#if __has_cpp_attribute(nodiscard)
+#  ifdef __clang__
+#    pragma GCC diagnostic ignored "-Wc++17-extensions"
+#  endif
+#  define NODISCARD [[nodiscard]]
+#elif __has_cpp_attribute(gnu::warn_unused_result)
+#  define NODISCARD [[gnu::warn_unused_result]]
+#else
+#  define NODISCARD
+#endif
+
+#endif // CPROVER_UTIL_NODISCARD_H