Enforce LWG DR 2566 requirement for container adaptors

Enforce LWG DR 2566 requirement for container adaptors

Although there is no good use for stack<int, deque<double>> or similar
types with a mismatched value_type, it's possible somebody is doing that
and getting away with it currently. This patch only enforces the new
requirement for C++17 and later. During stage 1 we should consider
enforcing it for C++11 and C++14.

	* doc/xml/manual/intro.xml: Document LWG 2566 status.
	* include/bits/stl_queue.h (queue, priority_queue): Add static
	assertions to enforce LWG 2566 requirement on value_type.
	* include/bits/stl_stack.h (stack): Likewise.

From-SVN: r268877