Core/Spells: Add GetTriggeringSpell() method to SpellScript and use it to properly filter server side spell 18350's script.

src/server/game/Spells/SpellScript.h

@@ -320,6 +320,9 @@ class SpellScript : public _SpellScript
         // Creates item. Calls Spell::DoCreateItem method.
         void CreateItem(uint32 effIndex, uint32 itemId);
 
+        // Returns SpellInfo from the spell that triggered the current one
+        SpellInfo const* GetTriggeringSpell();
+
         // finishes spellcast prematurely with selected error message
         void FinishCast(SpellCastResult result);