Fix some _GLIBCXX_DEBUG pretty printer errors

[François Dumont]

On 08/03/2018 19:11, Jonathan Wakely wrote:
> This caused a new regression for:
>
>    std::forward_list<std::string> flst;
>    std::forward_list<std::string>::iterator flstiter0;
> // { dg-final { note-test flstiter0 {non-dereferenceable iterator for
> std::forward_list}} }
>
>
> $1 = {_M_node = 0x0}
> got: $1 = {_M_node = 0x0}
> FAIL: libstdc++-prettyprinters/debug_cxx11.cc print flstiter0
>
Yes, sorry, it looks like I was more concerned by the fixes than by the 
errors on the new test.

This is coming from an attempt to move the forward_list iterators 
outside the std namespace. I'll provide a fix tomorrow.

François