libstdc++: Fix std::to_address for debug iterators (PR 93960)

libstdc++: Fix std::to_address for debug iterators (PR 93960)

It should be valid to use std::to_address on a past-the-end iterator,
but the debug mode iterators do a check for dereferenceable in their
operator->(). That check is generally useful, so rather than remove it
this changes std::__to_address to identify a debug mode iterator and
use base().operator->() to skip the check.

Backport from mainline
2020-04-03  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/93960
	* include/bits/ptr_traits.h (__to_address): Add special case for debug
	iterators, to avoid dereferenceable check.
	* testsuite/20_util/to_address/1_neg.cc: Adjust dg-error line number.
	* testsuite/20_util/to_address/debug.cc: New test.