libstdc++: Add [[nodiscard]] to sequence containers

libstdc++: Add [[nodiscard]] to sequence containers

... and container adaptors.

This adds the [[nodiscard]] attribute to functions with no side-effects
for the sequence containers and their iterators, and the debug versions
of those containers, and the container adaptors,

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/forward_list.h: Add [[nodiscard]] to functions
	with no side-effects.
	* include/bits/stl_bvector.h: Likewise.
	* include/bits/stl_deque.h: Likewise.
	* include/bits/stl_list.h: Likewise.
	* include/bits/stl_queue.h: Likewise.
	* include/bits/stl_stack.h: Likewise.
	* include/bits/stl_vector.h: Likewise.
	* include/debug/deque: Likewise.
	* include/debug/forward_list: Likewise.
	* include/debug/list: Likewise.
	* include/debug/safe_iterator.h: Likewise.
	* include/debug/vector: Likewise.
	* include/std/array: Likewise.
	* testsuite/23_containers/array/creation/3_neg.cc: Use
	-Wno-unused-result.
	* testsuite/23_containers/array/debug/back1_neg.cc: Cast result
	to void.
	* testsuite/23_containers/array/debug/back2_neg.cc: Likewise.
	* testsuite/23_containers/array/debug/front1_neg.cc: Likewise.
	* testsuite/23_containers/array/debug/front2_neg.cc: Likewise.
	* testsuite/23_containers/array/debug/square_brackets_operator1_neg.cc:
	Likewise.
	* testsuite/23_containers/array/debug/square_brackets_operator2_neg.cc:
	Likewise.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.
	* testsuite/23_containers/deque/cons/clear_allocator.cc: Cast
	result to void.
	* testsuite/23_containers/deque/debug/invalidation/4.cc:
	Likewise.
	* testsuite/23_containers/deque/types/1.cc: Use
	-Wno-unused-result.
	* testsuite/23_containers/list/types/1.cc: Cast result to void.
	* testsuite/23_containers/priority_queue/members/7161.cc:
	Likewise.
	* testsuite/23_containers/queue/members/7157.cc: Likewise.
	* testsuite/23_containers/vector/59829.cc: Likewise.
	* testsuite/23_containers/vector/ext_pointer/types/1.cc:
	Likewise.
	* testsuite/23_containers/vector/ext_pointer/types/2.cc:
	Likewise.
	* testsuite/23_containers/vector/types/1.cc: Use
	-Wno-unused-result.