Adds solidity::util::unreachable() helper function.

libsolutil/Assertions.h

@@ -40,6 +40,28 @@ namespace solidity::util
 #define ETH_FUNC __func__
 #endif
 
+#if defined(__GNUC__)
+// GCC 4.8+, Clang, Intel and other compilers compatible with GCC (-std=c++0x or above)
+[[noreturn]] inline __attribute__((always_inline)) void unreachable()
+{
+	__builtin_unreachable();
+}
+
+#elif defined(_MSC_VER) // MSVC
+
+[[noreturn]] __forceinline void unreachable()
+{
+	__assume(false);
+}
+
+#else
+
+[[noreturn]] inline void unreachable()
+{
+	solThrow(Exception, "Unreachable");
+}
+#endif
+
 namespace assertions
 {