Pass IdString by value instead of by const reference.

When IdString refcounting was expensive, it made sense to pass it by const reference
instead of by value, to avoid refcount churn. Now that IdString is not refcounted,
it's slightly more efficient to pass it by value.

frontends/ast/simplify.cc

@@ -879,7 +879,7 @@ static void check_auto_nosync(AstNode *node)
 	}
 
 	// remove the attributes we've "consumed"
-	for (const RTLIL::IdString &str : attrs_to_drop) {
+	for (RTLIL::IdString str : attrs_to_drop) {
 		auto it = node->attributes.find(str);
 		node->attributes.erase(it);
 	}