ada: Spurious error on formals with First_Controlling_Parameter type

This patch fixes a spurious error reported by the frontend when
a dispatching primitive of a tagged type has additional formals
with types that have the First_Controlling_Parameter aspect.

gcc/ada/ChangeLog:

	* sem_disp.adb (Check_Controlling_Formals): Formals of a type
	specifying aspect First_Controlling_Parameter are not candidate
	controlling parameters when they are not the first formal of
	the dispatching primitive.