All in all this is a bit inane, but the spec requires a zero-sized tuple
to provide allocator overloads for constructors, even though they do
absolutely nothing (they completely ignore the allocator). Presumably
these are also useful for some metaprogrammers I haven't heard of.
This takes us one small step further to pass libc++'s testsuite for tuple.

Tested on Linux-X64.

2016-06-05  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Support allocators in tuples of zero size.
    * include/std/tuple (tuple<>::tuple(),
    tuple<>::tuple(allocator_arg_t, const _Alloc&),
    tuple<>::tuple(allocator_arg_t, const _Alloc&, const tuple&),
    tuple<>::tuple(allocator_arg_t, const _Alloc&, tuple&&)): New.
    * testsuite/20_util/tuple/cons/allocators.cc: Adjust.