Implement DR resolutions for filesystem::copy

Implement DR resolutions for filesystem::copy

	* src/filesystem/ops.cc (do_copy_file): Return an error if either
	source or destination is not a regular file.
	(copy): Update comment to refer to LWG 2681. Implement 2682 and 2683
	resolutions.
	(read_symlink): Add missing ec.clear().
	* 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.
	* testsuite/experimental/filesystem/operations/read_symlink.cc: New.

From-SVN: r241484