libstdc++: Specialize std::pointer_traits<__normal_iterator<I,C>>

libstdc++: Specialize std::pointer_traits<__normal_iterator<I,C>>

This allows std::__to_address to be used with __normal_iterator in
C++11/14/17 modes. Without the partial specialization the deduced
pointer_traits::element_type is incorrect, and so the return type of
__to_address is wrong.

A similar partial specialization is probably needed for
__gnu_debug::_Safe_iterator.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (pointer_traits): Define partial
	specialization for __normal_iterator.
	* testsuite/24_iterators/normal_iterator/to_address.cc: New test.