libstdc++: Fix more missing uses of iter_difference_t [PR119820]

libstdc++: Fix more missing uses of iter_difference_t [PR119820]

libstdc++-v3/ChangeLog:

	PR libstdc++/119820
	* include/bits/ranges_algo.h (__shuffle_fn): Use
	ranges::distance to get difference type value to add to
	iterator.
	* include/std/format (__formatter_str::_M_format_range):
	Use ranges::next to increment iterator by a size_t value.

Reviewed-by: Patrick Palka <ppalka@redhat.com>