Avoid creating dangling references in case of nested tuples

Avoid creating dangling references in case of nested tuples
for tuple constructors that construct from other tuples.

2015-09-25  Ville Voutilainen  <ville.voutilainen@gmail.com>

	Avoid creating dangling references in case of nested tuples
	for tuple constructors that construct from other tuples.
	* include/std/tuple (_TC::_NonNestedTuple): New.
	* include/std/tuple (tuple::_TNTC): New.
	* include/std/tuple (tuple(const tuple<_UElements...>&),
	tuple(tuple<_UElements...>&&): Use _TNTC.
	* testsuite/20_util/tuple/cons/nested_tuple_construct.cc: New.

From-SVN: r228134