Implement LWG Paper n3887: Consistent Metafunction Aliases. This adds std::tuple_element_t<> as an alias for tuple_element<>::type. Clean up the synopsis for tuple_element in <utility> as well.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202673 91177308-0d34-0410-b5e6-96231b3b80d8