ada: Skip elaboration checks for abstract subprograms on derived types

Elaboration checks skip abstract subprogram declarations, which have no
body that could be examined. Now these checks also skip abstract
subprograms of a derived type, which have no body either.

gcc/ada/

	* sem_elab.adb (Check_Overriding_Primitive): Prevent Corresponding_Body
	to be called with entity of an abstract subprogram.