[Ada] Issue a warning on entity hidden in use_clause with -gnatwh

[Ada] Issue a warning on entity hidden in use_clause with -gnatwh

Augment the warnings issued with switch -gnatwh, so that a warning is
also issued when an entity from the package of a use_clause ends up
hidden due to an existing visible homonym.

gcc/ada/

	* sem_ch8.adb (Use_One_Package): Possibly warn.
	* sem_util.adb (Enter_Name): Factor out warning on hidden entity.
	(Warn_On_Hiding_Entity): Extract warning logic from Enter_Name and
	generalize it to be applied also on use_clause.
	* sem_util.ads (Warn_On_Hiding_Entity): Add new procedure.