Windows GDB: make windows_thread_info be private thread_info data

With Windows non-stop support, we'll add support for
target_thread_events to the Windows target.

When that is supported, and the core wants to be notified of thread
exit events, the target backend does not delete the thread for which
the event is being reported.  Instead, infrun takes care of that.

That causes one problem on Windows, which is that Windows maintains
its own separate Windows threads list, in parallel with the struct
thread_info thread list maintained by the core.  In the
target_thread_events events scenario, when infrun deletes the thread,
the corresponding object in the Windows backend thread list is left
dangling, causing problems.

Fix this by eliminating the parallel thread list from the Windows
backend, instead making the windows_thread_info data by registered as
the private data associated with thread_info, like other targets do.

It also adds a all_windows_threads walker function, and associated
range and iterator classes, so that most of the Windows target code
can iterate over Windows threads without having to worry about
fetching the Windows thread data out of thread_info's private data.

Change-Id: I5058969b46e0dd238c89b6c28403c1848f083683