Improve _GLIBCXX_DEBUG safe iterator range size computation.

Improve _GLIBCXX_DEBUG safe iterator range size computation.

	* include/debug/forward_list
	(_Sequence_traits<__debug::forward_list<>>::_S_size): Returns __dp_sign
	distance when not empty.
	* include/debug/list (_Sequence_traits<__debug::list<>>::_S_size):
	Likewise.
	* include/debug/helper_functions.h (__dp_sign_max_size): New
	_Distance_precision enum entry.
	(__valid_range_aux(_IIte, _IIte, _Distance_traits<>::__type,
	__false_type)): Adapt.
	* include/debug/safe_iterator.tcc
	(_Safe_iterator<>::_M_get_distance_to(const _Safe_iterator&)): Review
	distance computation.

From-SVN: r276600