LWG 2537 fix priority_queue constructors to establish invariant

LWG 2537 fix priority_queue constructors to establish invariant

This change is safe to make now (in stage 4), because the constructors
are currently incorrect and unusable (unless the supplied container
already contains a heap, in which case the new make_heap calls are
redundant but harmless).

	* doc/xml/manual/intro.xml: Document LWG 2537 status.
	* include/bits/stl_queue.h
	(priority_queue(const Compare&, const Container&, const Alloc&))
	(priority_queue(const Compare&, Container&&, const Alloc&)): Call
	make_heap.
	* testsuite/23_containers/priority_queue/dr2537.cc: New test.

From-SVN: r268878