Mark reductions as fragile based on their final properties

We previously maintained a set of individual productions that were
involved in conflicts, but that was subtly incorrect because
we don't compare productions themselves when comparing parse items;
we only compare the parse items properties that could affect the
final reduce actions.

test/runtime/parser_test.cc

@@ -292,7 +292,7 @@ describe("Parser", [&]() {
               "(number) "
               "(math_op (number) (math_op (number) (identifier)))))))");
 
-        AssertThat(input->strings_read(), Equals(vector<string>({"123 || 5 ", ";"})));
+        AssertThat(input->strings_read(), Equals(vector<string>({"123 || 5 "})));
       });
     });