ada: Follow-up adjustment after fix to Default_Initialize_Object

Now that Default_Initialize_Object honors the No_Initialization flag in all
cases, objects of an access type declared without initialization expression
can no longer be considered as being automatically initialized to null.

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Examine the Expression
	field after the call to Default_Initialize_Object in order to set
	Is_Known_Null, as well as Is_Known_Non_Null, on an access object.