Backport all Filesystem library fixes from trunk

Backport all Filesystem library fixes from trunk

This implements the following LWG DRs:
2681 2682 2683 2706 2707 2712 2720 2723

	Backport from mainline
	PR libstdc++/70975
	PR libstdc++/71337
	PR libstdc++/78111
	* include/experimental/bits/fs_dir.h (recursive_directory_iterator):
	Overload pop (LWG 2706).
	* include/experimental/bits/fs_fwd.h (perms::resolve_symlinks):
	Replace with symlink_nofollow (LWG 2720).
	* include/experimental/bits/fs_ops.h
	(exists(const path&, error_code&)): Clear error if status is known
	(LWG 2725).
	* include/experimental/bits/fs_path.h (__is_path_src)
	(_S_range_begin, _S_range_end): Overload to treat string_view as a
	Source object.
	(path::operator+=, path::compare): Overload for basic_string_view.
	(path::path(string_type&&))
	(path::operator=(string&&), path::assign(string_type&&)): Define
	construction and assignment from string_type rvalues (LWG 2707).
	(path::_S_convert<_Iter>(_Iter, _Iter)): Remove cv-qualifiers from
	iterator's value_type.
	(path::_S_convert<_Iter>(_Iter __first, __null_terminated)): Likewise.
	Do not use operation not supported by input iterators.
	(path::__is_path_iter_src): Add partial specialization for const
	encoded character types.
	* src/filesystem/dir.cc (open_dir): Return same value for errors
	whether ignored or not.
	(_Dir::advance(error_code*, directory_options)): Return false on
	error.
	(directory_iterator(const path&, directory_options, error_code*)):
	Create end iterator on error (LWG 2723).
	(recursive_directory_iterator(const path&, directory_options,
	error_code*)): Likewise.
	(recursive_directory_iterator::increment): Reset state on error.
	(recursive_directory_iterator::pop): Define new overload.
	* src/filesystem/ops.cc (canonical): Set error for non-existent path.
	(file_time): Take error_code parameter and check for overflow.
	(close_fd): Remove.
	(do_copy_file): Report an error if source or destination is not a
	regular file (LWG 2712). Pass error_code in file_time calls.  Just
	use close(3) instead of close_fd, to prevent retrying on error.
	Check if _GLIBCXX_USE_FCHMODAT is defined.
	[_GLIBCXX_USE_SENDFILE]: Fallback to read/write operations in case
	sendfile fails with ENOSYS or EINVAL. Pass non-null pointer to
	sendfile for offset argument.
	(copy): Update comment to refer to LWG 2681. Implement 2682 and 2683
	resolutions.
	(equivalent): Fix error handling and result when only one file exists.
	(is_empty): Fix error handling.
	(last_write_time(const path&, error_code&)): Pass error_code in
	file_time calls.
	(last_write_time(const path&, file_time_type, error_code&)): Handle
	negative times correctly.
	(permissions(const path&, perms, error_code&)): Handle
	symlink_nofollow.
	(read_symlink): Add missing ec.clear().
	(status(const path&, error_code&)): Handle EOVERFLOW.
	(temp_directory_path): Pass error_code argument to other filesystem
	operations.
	* testsuite/experimental/filesystem/iterators/directory_iterator.cc:
	Update expected behaviour on error.
	* testsuite/experimental/filesystem/iterators/pop.cc: New.
	* testsuite/experimental/filesystem/iterators/
	recursive_directory_iterator.cc: Update expected behaviour on error.
	* testsuite/experimental/filesystem/operations/copy.cc: Update
	expected behaviour for copying directories with create_symlinks.
	Verify that error_code arguments are cleared if there's no error.
	Remove files created by tests. Test copying directories.
	* testsuite/experimental/filesystem/operations/copy_file.cc: Remove
	files created by tests.
	* testsuite/experimental/filesystem/operations/create_symlink.cc: New.
	* testsuite/experimental/filesystem/operations/equivalent.cc: New.
	* testsuite/experimental/filesystem/operations/exists.cc: Test
	overload taking an error_code.
	* testsuite/experimental/filesystem/operations/is_empty.cc: New.
	* testsuite/experimental/filesystem/operations/last_write_time.cc:
	New.
	* testsuite/experimental/filesystem/operations/permissions.cc: Test
	overload taking error_code. Test symlink_nofollow on non-symlinks.
	* testsuite/experimental/filesystem/operations/read_symlink.cc: New.
	* testsuite/experimental/filesystem/operations/remove_all.cc: New.
	* testsuite/experimental/filesystem/operations/temp_directory_path.cc:
	Add testcase for inaccessible directory.
	* testsuite/experimental/filesystem/path/construct/range.cc: Test
	construction from input iterators with const value types.
	* testsuite/experimental/filesystem/path/construct/string_view.cc:
	New.
	* testsuite/util/testsuite_fs.h (scoped_file): Define RAII helper.

From-SVN: r243557