[libc++] Declare std::tuple_element as struct instead of class

Similarly to https://reviews.llvm.org/rL350972, this revision changes
std::tuple_element from class to struct.

Fixes PR41331.
Thanks to Jan Wilken Dörrie for the patch.

Differential Revision: https://reviews.llvm.org/D60069

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357411 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_element.fail.cpp

@@ -11,9 +11,8 @@
 // template <class... Types> class tuple;
 
 // template <size_t I, class... Types>
-// class tuple_element<I, tuple<Types...> >
+// struct tuple_element<I, tuple<Types...> >
 // {
-// public:
 //     typedef Ti type;
 // };