ada: Fix check for elaboration order on subprogram body stubs

ada: Fix check for elaboration order on subprogram body stubs

Fix an assertion failure occurring when elaboration checks were applied to
subprogram with a separate body.

gcc/ada/ChangeLog:

	* sem_elab.adb (Check_Overriding_Primitive): Find early call region
	of the subprogram body declaration, not of the subprogram body stub.