ada: Avoid marking a ghost pragma twice

ada: Avoid marking a ghost pragma twice

Previously all assertion level pragmas were marked based on the
current scope. Additionally some pragmas were marked afterwards
based on the relevant entity e.g. Pre. This would lead to cases
where a pragma was marked as both ignored and checked ghost pragma.

Each pragma should get marked only once based on their individual
semantics.

gcc/ada/ChangeLog:

	* sem_prag.adb (Analyze_Pragma): Set Mark_Ghost_Code individually
	based on the semantics of each pragma.