MDEV-7482 VIEW containing INTERVAL(...)

Item_func::print() prints itself as name + "(" + arguments + ")".
Normally that works, but Item_func_interval internally implements its
arguments as one single Item_row. Item_row prints itself as
"(" + values + ")". As a result, "INTERVAL(1,2)" was being printed
as "INTERVAL((1,2))". Fixed with a custom Item_func_interval::print().

sql/item_cmpfunc.h

@@ -734,6 +734,11 @@ public:
   void fix_length_and_dec();
   const char *func_name() const { return "interval"; }
   uint decimal_precision() const { return 2; }
+  void print(String *str, enum_query_type query_type)
+  {
+    str->append(func_name());
+    print_args(str, 0, query_type);
+  }
 };