ada: Small consistency fix for -gnatwv warning

ada: Small consistency fix for -gnatwv warning

The goal is to arrange for the warning to be issued consistently between
objects whose address is taken and objects whose address is not taken.

gcc/ada/

	* sem_warn.adb (Check_References.Type_OK_For_No_Value_Assigned):
	New predicate.
	(Check_References): For Warn_On_No_Value_Assigned, use the same test
	on the type in the address-not-taken and default cases.

gcc/testsuite/ChangeLog:

	* gnat.dg/warn25.adb: Add xfail.