ada: Fix inefficient Unchecked_Conversion to large array type

We fail to use the implementation permission given by RM 13.9(12) because
the array type does not have the Size_Known_At_Compile_Time flag set.

gcc/ada/ChangeLog:

	* freeze.adb (Check_Compile_Time_Size): Try harder to see whether
	the bounds of array types are known at compile time.