libstdc++: Make more _Safe_iterator functions in constexpr in C++20.

libstdc++: Make more _Safe_iterator functions in constexpr in C++20.

This functions are indirectly called from flat_ containers operations
(from preconditions check of lower_bound, upper_bound, ...) that were
made constexpr by r16-6026-gbf9dd44a97400e, leading to test with
in _GLIBCXX_DEBUG mode.

For __can_advance we uncoditionally return true in constant evaluation,
similary to __valid_range. The constexpr iterator will detect comparision
of iterator to different ranges.

libstdc++-v3/ChangeLog:

	* include/debug/helper_functions.h (__gnu_debug::__can_advance):
	Declare as _GLIBCXX20_CONSTEXPR.
	* include/debug/safe_iterator.h (__gnu_debug::__can_advance):
	Define as _GLIBCXX20_CONSTEXPR, and return true for constexpr
	evaluation.
	(__gnu_debug::__base): Define as _GLIBCXX20_CONSTEXPR.