Implement the proposed resolution of LWG 2708 by moving the _M_options
and _M_pending members out of the recursive_directory_iterator into the
shared _Dir_stack object. Because _Dir_stack is an opaque type, the
member functions that access the _M_options and _M_pending variables
cannot be inline. Move them into the library.

As a drive-by fix, add noexcept to the non-throwing member functions of
recursive_directory_iterator.

	* config/abi/pre/gnu.ver: Export new symbols.
	* include/bits/fs_dir.h (recursive_directory_iterator::options())
	(recursive_directory_iterator::recursion_pending())
	(recursive_directory_iterator::disable_recursion_pending()): Remove
	inline definitions. Make noexcept.
	(recursive_directory_iterator::depth())
	(recursive_directory_iterator::operator*())
	(recursive_directory_iterator::operator->()): Make noexcept.
	(recursive_directory_iterator::_M_options)
	(recursive_directory_iterator::_M_pending): Remove data members.
	* src/c++17/fs_path.cc (_Dir_stack): Add constructor and data members.
	(recursive_directory_iterator::recursive_directory_iterator): Remove
	ctor-initializer. Use new constructor for _Dir_stack.
	(recursive_directory_iterator::options())
	(recursive_directory_iterator::recursion_pending())
	(recursive_directory_iterator::disable_recursion_pending()): Add
	non-inline definitions.
	(recursive_directory_iterator::depth()): Make noexcept.
	(recursive_directory_iterator::increment(error_code&))
	(recursive_directory_iterator::pop(error_code&)): Adjust to new
	location of options and recursion_pending members.
	* testsuite/27_io/filesystem/iterators/recursion_pending.cc: New test.
	* testsuite/util/testsuite_fs.h (__gnu_test::scoped_file): Add
	user-declared move constructor and assignment operator, to make the
	type move-only.

From-SVN: r270173