This patch attempts to improve the QoI of std::tuples tuple_element and
__make_tuple_types helpers. Previously they required O(N) instantiations,
one for every element in the tuple
The new implementations are O(1) after __tuple_indices<Id...> is created.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@274330 91177308-0d34-0410-b5e6-96231b3b80d8
