libstdc++: Reset filesystem::recursive_directory_iterator on error

libstdc++: Reset filesystem::recursive_directory_iterator on error

The standard requires directory iterators to become equal to the end
iterator value if they report an error. Some members functions of
filesystem::recursive_directory_iterator fail to do that.

libstdc++-v3/ChangeLog:

	* src/c++17/fs_dir.cc (recursive_directory_iterator::increment):
	Reset state to past-the-end iterator on error.
	(fs::recursive_directory_iterator::pop(error_code&)): Likewise.
	(fs::recursive_directory_iterator::pop()): Check _M_dirs before
	it might get reset.
	* src/filesystem/dir.cc (recursive_directory_iterator): Likewise,
	for the TS implementation.
	* testsuite/27_io/filesystem/iterators/error_reporting.cc: New test.
	* testsuite/experimental/filesystem/iterators/error_reporting.cc: New test.