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.
fa98bc4270