libstdc++: Qualify calls to debug mode helpers

These functions should be qualified to disable unwanted ADL.

The overload of __check_singular_aux for safe iterators was previously
being found by ADL, because it wasn't declared before __check_singular.
Add a declaration so that it can be found by qualified lookup.

libstdc++-v3/ChangeLog:

	* include/debug/helper_functions.h (__get_distance)
	(__check_singular, __valid_range_aux, __valid_range): Qualify
	calls to disable ADL.
	(__check_singular_aux(const _Safe_iterator_base*)): Declare
	overload that was previously found via ADL.