libstdc++/70609 fix filesystem::copy()

libstdc++/70609 fix filesystem::copy()

	PR libstdc++/70609
	* src/filesystem/ops.cc (close_fd): New function.
	(do_copy_file): Set permissions before copying file contents. Check
	result of closing file descriptors. Don't copy streambuf when file
	is empty.
	(copy(const path&, const path&, copy_options, error_code&)): Use
	lstat for source file when copy_symlinks is set.
	* testsuite/experimental/filesystem/operations/copy.cc: Test copy().

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@235215 138bc75d-0d04-0410-961f-82ee72b054a4