[Ada] Simplify iteration over formal parameters for Global/Depends check

[Pierre-Marie de Rodat]

Replace low-level First_Entity/Next_Entity with a high-level
First_Formal/Next_Formal. The new code is easier to debug, because we
don't even see local subprograms, objects, etc.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_prag.adb (Collect_Global_Item): Iterate directly over
	formals.
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