ada: Fix incorrect warning about unreferenced packed arrays

ada: Fix incorrect warning about unreferenced packed arrays

This patch fixes a bug in which a reference to a renaming of a
component of a packed array was not counted as a reference,
and thus caused incorrect warnings about unreferenced objects.

gcc/ada/

	* sem_ch5.adb (Analyze_Assignment): Fix the bug by checking
	Original_Node. The renaming might be elsewhere, but the (original)
	reference is right here.
	* errout.adb: Remove pragma Unreferenced which was added because
	of the above bug.
	* einfo.ads: Misc cleanup.
	* lib.adb: Likewise.
	* lib.ads: Likewise.