[libcxx] [test] Fix -Wmismatched-tags in tuple_size_structured_bindings.pass.cpp.

Clang and C1XX both complain about mismatched class/struct, but libc++ and MSVC's STL
differ on what they use for tuple_element/tuple_size, so there's no way to win here.

I'm reverting this part of my previous change. In the future, I'll have to suppress
the warning for one compiler or the other.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305854 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_structured_bindings.pass.cpp

@@ -117,7 +117,8 @@ template <size_t N>
 int get(Test const&) { static_assert(N == 0, ""); return -1; }
 
 template <>
-struct std::tuple_element<0, Test> {
+class std::tuple_element<0, Test> {
+public:
   typedef int type;
 };
 
@@ -128,7 +129,8 @@ void test_before_tuple_size_specialization() {
 }
 
 template <>
-struct std::tuple_size<Test> {
+class std::tuple_size<Test> {
+public:
   static const size_t value = 1;
 };