Add backpointer from windows_thread_info to windows_process_info

The next patch will move some duplicated code in gdb and gdbserver to
gdb/nat/windows-nat.c, where it would be convenient to get at the
Windows process info of a given Windows thread info, from within a
windows_thread_info method.

I first thought of passing down the windows_process_info pointer as
argument to the windows_thread_info method, but that looked a bit odd.
I think it looks better to just add a back pointer, so that's what
this patch does.  The following patch will then add a use of it.

I suspect this will help moving more duplicated code to
gdb/nat/windows-nat.c in the future, too.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I47fc0d3323be5b6f6fcfe912b768051a41910666