Make save_infcall_*_state return unique pointers

Make save_infcall_*_state return unique pointers

Simon pointed out that save_infcall_suspend_state and
save_infcall_control_state could return unique pointers.  This patch
implements this idea.

gdb/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* infrun.c (save_infcall_suspend_state): Return
	infcall_suspend_state_up.
	(save_infcall_control_state): Return infcall_control_state_up.
	* inferior.h (save_infcall_suspend_state)
	(save_infcall_control_state): Declare later.  Return unique
	pointers.