ada: Array aggregate with large static bounds causes compiler crash

In some cases an array aggregate with statically known bounds and at least
one bound outside of the range of a 32-bit signed integer causes
a bugbox.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Convert_To_Positional.Flatten): Avoid raising
	Constraint_Error in UI_To_Int by testing UI_Is_In_Int_Range first.