ada: Move checks for consequences of Exceptional_Cases to GNAT

Previously checks for consequence expressions of Exceptional_Cases aspects were
done in GNATprove backend. However, we can do them in the frontend, where they
will apply to all subprograms, regardless of the SPARK_Mode aspect.

gcc/ada/ChangeLog:

	* sem_prag.adb (Analyze_Exceptional_Cases_In_Decl_Part): Move check
	from GNATprove backend to GNAT frontend.