ada: Fix pragma Import ignored in conjunction with address aspect

The special mechanism used to "deinitialize" an object in the presence of
a pragma Import works in conjunction with an address clause, but not with
an address aspect.  This tweaks the expansion so as to plug this loophole.

gcc/ada/ChangeLog:

	* einfo.ads (Initialization_Statements): Adjust description.
	* exp_ch3.adb (Default_Initialize_Object): Use the special mechanism
	of Initialization_Statements, if need be, instead of inserting the
	initialization statements directly into the freeze actions.