[patch] Simplify std::tuple helpers and fix C++14 bug.

[Jonathan Wakely]

I found a bug in the new std::get<Tp>(tuple<Types...>&&) function
where it didn't compile when trying to access reference member.

The rest of this patch simplifies the code in <tuple> by:

 - removing the redundant __add_ref etc. helpers.
 - defining a __tuple_element_t alias template.
 - removing duplication in the tuple_size<cv T> traits.
 - using static_assert in the relational operators.

Tested x86_64-linux, committed to trunk.

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