LWG #2212 (not yet adopted) mandates that tuple_size/tuple_element are available if <array> or <utility> are included (not just <tuple>). We already do this. Add some tests to make sure that this remains true.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220295 91177308-0d34-0410-b5e6-96231b3b80d8