ada: Fix internal error on Ghost aspect applied to Big_Integers

That's a regression introduced by the rewrite of the finalization machinery,
in the form of dangling references to Master_Node entities remaining in the
tree after the removal of the ignored Ghost code.

gcc/ada/ChangeLog:

	* exp_ch7.adb (Process_Transient_In_Scope): Bail out if the object
	is an ignored ghost entity.