Code cleanup related to examining uses of Check_Unset_Reference for
improved detection of uninitialised scalar objects. Semantics is
unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_util.adb (Aggregate_Constraint_Checks): Fix whitespace;
	refactor repeated code; replace a ??? comment with an
	explanation based on the comment for the routine spec.