ada: First controlling parameter: report error without Extensions allowed

Enable reporting an error when this new aspect/pragma is set to
True, and the sources are compiled without language extensions
allowed.

gcc/ada/

	* sem_ch13.adb (Analyze_One_Aspect): Call
	Error_Msg_GNAT_Extension() to report an error when the aspect
	First_Controlling_Parameter is set to True and the sources are
	compiled without Core_Extensions_ Allowed.
	* sem_prag.adb (Pragma_First_Controlling_Parameter): Call
	subprogram Error_Msg_GNAT_Extension() to report an error when the
	aspect First_Controlling_Parameter is set to True and the sources
	are compiled without Core_Extensions_Allowed. Report an error when
	the aspect pragma does not confirm an inherited True value.