libstdc++: Overload std::__to_address for __gnu_cxx::__normal_iterator.

Prefer to overload __to_address to partially specialize std::pointer_traits because
std::pointer_traits would be mostly useless. Moreover partial specialization of
pointer_traits<__normal_iterator<P, C>> fails to rebind C, so you get incorrect types
like __normal_iterator<long*, vector<int>>. In the case of __gnu_debug::_Safe_iterator
the to_pointer method is impossible to implement correctly because we are missing
the parent container to associate the iterator to.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h
	(std::pointer_traits<__gnu_cxx::__normal_iterator<>>): Remove.
	(std::__to_address(const __gnu_cxx::__normal_iterator<>&)): New for C++11 to C++17.
	* include/debug/safe_iterator.h
	(std::__to_address(const __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<>,
	_Sequence>&)): New for C++11 to C++17.
	* testsuite/24_iterators/normal_iterator/to_address.cc: Add check on std::vector::iterator
	to validate both __gnu_cxx::__normal_iterator<> __to_address overload in normal mode and
	__gnu_debug::_Safe_iterator in _GLIBCXX_DEBUG mode.