test: squelch -Wreturn-type error

Add an unreachable marker to avoid a -Wreturn-type error when building
on Windows.

llvm-svn: 294901

libcxx/test/std/utilities/function.objects/func.not_fn/not_fn.pass.cpp

@@ -414,7 +414,14 @@ void throws_in_constructor_test()
         throw 42;
       }
       ThrowsOnCopy() = default;
-      bool operator()() const { assert(false); }
+      bool operator()() const {
+        assert(false);
+#if defined(_LIBCPP_MSVC)
+        __assume(0);
+#else
+        __builtin_unreachable();
+#endif
+      }
     };
     {
         ThrowsOnCopy cp;