Tolerate [[nodiscard]] annotations in the STL. Reviewed as https://reviews.llvm.org/D39033

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318276 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/optional/optional.object/optional.object.observe/value_const.pass.cpp

@@ -61,7 +61,7 @@ int main()
         const optional<X> opt;
         try
         {
-            opt.value();
+            (void)opt.value();
             assert(false);
         }
         catch (const bad_optional_access&)