PR libstdc++/86734 make reverse_iterator::operator-> more robust

PR libstdc++/86734 make reverse_iterator::operator-> more robust

Implement the proposed resolution from LWG 1052, which also resolves
DR 2118 by avoiding taking the address in the first place.

	PR libstdc++/86734
	* include/bits/stl_iterator.h (reverse_iterator::operator->): Call
	_S_to_pointer (LWG 1052, LWG 2118).
	(reverse_iterator::_S_to_pointer): Define overloaded helper functions.
	* testsuite/24_iterators/reverse_iterator/dr1052.cc: New test.
	* testsuite/24_iterators/reverse_iterator/dr2188.cc: New test.

From-SVN: r263074