[v3 PATCH] Protect allocator-overloads of tuple-from-tuple constructors from cases that would create dangling references.

[Ville Voutilainen <ville dot voutilainen at gmail dot com>]

The fix to avoid binding dangling references to temporaries for tuple's
constructors that take tuples of different type didn't include the fix
for allocator overloads. That was just lazy, and I should feel ashamed.
This patch fixes it, and takes us one step further to pass libc++'s testsuite
for tuple. The added _NonNestedTuple checks could actually be folded
into the recently-added _TMCT alias, but I'll do that as a separate cleanup
patch. For now, this should do as an easy and straightforward fix.

Tested on Linux-x64.

2016-05-28  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Protect allocator-overloads of tuple-from-tuple constructors
    from cases that would create dangling references.
    * include/std/tuple (tuple(allocator_arg_t, const _Alloc&,
     const tuple<_UElements...>&), tuple(allocator_arg_t, const _Alloc&,
     tuple<_UElements...>&&)): Add a check for _NonNestedTuple.
    * testsuite/20_util/tuple/cons/nested_tuple_construct.cc: Adjust.

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