PR77528 partially revert r244278 and define default constructors

PR77528 partially revert r244278 and define default constructors

	PR libstdc++/77528
	* include/bits/stl_queue.h (queue, priority_queue): Remove default
	member-initializers and define default constructors as templates with
	constraints.
	* include/bits/stl_stack.h (stack): Likewise.
	* testsuite/23_containers/priority_queue/requirements/constructible.cc:
	New.
	* testsuite/23_containers/priority_queue/requirements/
	explicit_instantiation/1.cc: Test more instantiations.
	* testsuite/23_containers/priority_queue/requirements/
	explicit_instantiation/1_c++98.cc: Likewise.
	* testsuite/23_containers/queue/requirements/constructible.cc: New.
	* testsuite/23_containers/stack/requirements/constructible.cc: New.

From-SVN: r244374