Implement P0084R2, Emplace return type, for C++17

Implement P0084R2, Emplace return type, for C++17

	* doc/xml/manual/status_cxx2017.xml: Update status.
	* doc/html/*: Regenerate.
	* include/bits/deque.tcc (deque::emplace_front, deque::emplace_back):
	Return a reference in C++17 mode.
	* include/bits/forward_list.h (forward_list::emplace_front): Likewise.
	* include/bits/stl_bvector.h (vector<bool>::emplace_back): Likewise.
	* include/bits/stl_deque.h (deque::emplace_front, deque::emplace_back):
	Likewise.
	* include/bits/stl_list.h (list::emplace_front, list::emplace_back):
	Likewise.
	* include/bits/stl_queue.h (queue::emplace): Likewise.
	* include/bits/stl_stack.h (stack::emplace): Likewise.
	* include/bits/stl_vector.h (vector::emplace_back): Likewise.
	* include/bits/vector.tcc (vector::emplace_back): Likewise.
	* include/debug/deque (__gnu_debug::deque::emplace_front)
	(__gnu_debug::deque::emplace_back): Likewise.
	* include/debug/vector (__gnu_debug::vector::emplace_back): Likewise.
	* testsuite/23_containers/deque/modifiers/emplace/cxx17_return.cc:
	New.
	* testsuite/23_containers/forward_list/modifiers/
	emplace_cxx17_return.cc: New.
	* testsuite/23_containers/list/modifiers/emplace/cxx17_return.cc: New.
	* testsuite/23_containers/queue/members/emplace_cxx17_return.cc: New.
	* testsuite/23_containers/stack/members/emplace_cxx17_return.cc: New.
	* testsuite/23_containers/vector/bool/emplace_cxx17_return.cc: New.
	* testsuite/23_containers/vector/modifiers/emplace/cxx17_return.cc:
	New.

From-SVN: r241234