Implement resolution of LWG DR 685 precisely

Implement resolution of LWG DR 685 precisely

	PR libstdc++/71771
	* include/bits/stl_iterator.h
	(operator-(reverse_iterator<Iter>, reverse_iterator<Iter>): Only
	define for C++98 mode.
	(operator-(move_iterator<Iter>, move_iterator<Iter>): Don't define.
	* testsuite/24_iterators/headers/iterator/synopsis.cc: Use
	-std=gnu++98.
	* testsuite/24_iterators/headers/iterator/synopsis_c++11.cc: New test.
	* testsuite/24_iterators/headers/iterator/synopsis_c++14.cc: New test.
	* testsuite/24_iterators/headers/iterator/synopsis_c++17.cc: New test.
	* testsuite/24_iterators/move_iterator/greedy_ops.cc: Don't test
	difference operator.
	* testsuite/24_iterators/reverse_iterator/greedy_ops.cc: Only test
	difference operator for C++98.
	* testsuite/24_iterators/reverse_iterator/71771.cc: New test.

From-SVN: r239691