libstdc++: Fix iota_view::size() to avoid overflow

libstdc++: Fix iota_view::size() to avoid overflow

This avoids the overflow that occurs when negating the most negative
value of an integral type.

Also prevent returning signed int when the values have lower rank and
promote to int.

libstdc++-v3/ChangeLog:

	* include/std/ranges (ranges::iota_view::size()): Perform all
	calculations in the right unsigned types.
	* testsuite/std/ranges/iota/size.cc: New test.