Placate unused variable warnings uncovered by improvements to clang's -Wunused-variable

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@315809 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp

@@ -49,6 +49,7 @@ int main()
 {
     {
         std::tuple<> t;
+        (void)t;
     }
     {
         std::tuple<int> t;
@@ -88,6 +89,7 @@ int main()
     }
     {
         constexpr std::tuple<> t;
+        (void)t;
     }
     {
         constexpr std::tuple<int> t;