Implement TranslationUnit.ToString() to help with debugging.

Author: tritao

src/AST/TranslationUnit.cs

@@ -83,5 +83,7 @@ public override T Visit<T>(IDeclVisitor<T> visitor)
         {
             return visitor.VisitTranslationUnit(this);
         }
+
+        public override string ToString() => FileName;
     }
 }