ada: Tune recent change for bit-packed arrays to help GNATprove backend

When GNAT is operating in GNATprove_Mode the Expander_Active flag is disabled,
but we still must do things that ordinary backends expect.

gcc/ada/ChangeLog:

	* sem_util.adb (Get_Actual_Subtype): Do the same for GCC and GNATprove
	backends.