Use lwp, not tid, for Windows thread id

Use lwp, not tid, for Windows thread id

This changes windows-nat.c to put the Windows thread id into the "lwp"
field of ptid_t, not the "tid" field.  This is done for two reasons.

First, ptid.h has this to say:

   process_stratum targets that handle threading themselves should
   prefer using the ptid.lwp field, leaving the ptid.tid field for any
   thread_stratum target that might want to sit on top.

Second, this change brings gdb and gdbserver into sync here, which
makes sharing code simpler.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_add_thread, windows_delete_thread)
	(windows_nat_target::fetch_registers)
	(windows_nat_target::store_registers, fake_create_process)
	(windows_nat_target::resume, windows_nat_target::resume)
	(get_windows_debug_event, windows_nat_target::wait)
	(windows_nat_target::pid_to_str)
	(windows_nat_target::get_tib_address)
	(windows_nat_target::get_ada_task_ptid)
	(windows_nat_target::thread_name)
	(windows_nat_target::thread_alive): Use lwp, not tid.