SIL: Fix an old FIXME

Author: slavapestov

lib/SIL/SILFunctionType.cpp

@@ -983,10 +983,8 @@ class DestructureInputs {
         for (auto i : indices(substTupleTy.getElementTypes())) {
           auto &elt = substTupleTy->getElement(i);
           auto ownership = elt.getParameterFlags().getValueOwnership();
-          // FIXME(swift3): Once the entire parameter list is no longer a
-          // target for substitution, re-enable this.
-          // assert(ownership == ValueOwnership::Default);
-          // assert(!elt.isVararg());
+          assert(ownership == ValueOwnership::Default);
+          assert(!elt.isVararg());
           visit(ownership, forSelf,
                 origType.getTupleElementType(i),
                 CanType(elt.getRawType()), rep);