Don't write to inferior_ptid in infrun.c

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* infrun.c (generic_mourn_inferior): Use switch_to_thread instead
	of writing to inferior_ptid.
	(scoped_restore_exited_inferior): Delete.
	(handle_vfork_child_exec_or_exit): Simplify using
	scoped_restore_current_pspace_and_thread.  Use switch_to_thread
	instead of writing to inferior_ptid.
	(THREAD_STOPPED_BY): Delete.
	(thread_stopped_by_watchpoint, thread_stopped_by_sw_breakpoint)
	(thread_stopped_by_hw_breakpoint): Delete.
	(save_waitstatus): Use
	scoped_restore_current_thread+switch_to_thread, and call
	target_stopped_by_watchpoint instead of
	thread_stopped_by_watchpoint, target_stopped_by_sw_breakpoint
	instead of thread_stopped_by_sw_breakpoint, and
	target_stopped_by_hw_breakpoint instead of
	thread_stopped_by_hw_breakpoint.
	(handle_inferior_event)
	<TARGET_WAITKIND_EXITED/TARGET_WAITKIND_SIGNALLED>: Don't write to
	inferior_ptid directly, nor
	set_current_inferior/set_current_program_space.  Use
	switch_to_thread / switch_to_inferior_no_thread instead.