Add nodiscard utility

This just substitutes for `[[nodiscard]]` until we use C++17 -- most modern GNU or
GNU-compatible compilers will give us a similar utility in the meantime.

Author: smowton

src/util/nodiscard.h

@@ -0,0 +1,26 @@
+/*******************************************************************\
+
+Module: No-discard-result macro
+
+Author: Diffblue Ltd.
+
+\*******************************************************************/
+
+/// \file
+/// No-discard-result macro
+
+#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]] // NOLINT
+#elif __has_cpp_attribute(gnu::warn_unused_result)
+#  define NODISCARD [[gnu::warn_unused_result]] // NOLINT
+#else
+#  define NODISCARD
+#endif
+
+#endif