cleanup: Arg_comparator::cache_converted_constant()

It's a generic function, not using anything from Arg_comparator.
Make it a static function, not a class method, to be able to use
it later without Arg_comparator

sql/item_cmpfunc.cc

@@ -34,6 +34,8 @@
 #include "sql_time.h"                  // make_truncated_value_warning
 #include "sql_base.h"                  // dynamic_column_error_message
 
+static Item** cache_converted_constant(THD *thd, Item **value,
+                Item **cache_item, Item_result type,enum_field_types f_type);
 
 /**
   find an temporal type (item) that others will be converted to
@@ -707,9 +709,8 @@ int Arg_comparator::set_cmp_func(Item_func_or_sum *owner_arg,
   @return cache item or original value.
 */
 
-Item** Arg_comparator::cache_converted_constant(THD *thd, Item **value,
-                                                Item **cache_item,
-                                                Item_result type)
+static Item** cache_converted_constant(THD *thd, Item **value,
+                                       Item **cache_item, Item_result type)
 {
   /*
     Don't need cache if doing context analysis only.

sql/item_cmpfunc.h

@@ -113,8 +113,6 @@ public:
   int compare_e_json_str();
   int compare_e_str_json();
 
-  Item** cache_converted_constant(THD *thd, Item **value, Item **cache,
-                                  Item_result type);
   static arg_cmp_func comparator_matrix [6][2];
   inline bool is_owner_equal_func()
   {
@@ -1069,7 +1067,7 @@ class Item_func_nullif :public Item_func_hybrid_field_type
     The left "a" is in a comparison and can be replaced by:
     - Item_func::convert_const_compared_to_int_field()
     - agg_item_set_converter() in set_cmp_func()
-    - Arg_comparator::cache_converted_constant() in set_cmp_func()
+    - cache_converted_constant() in set_cmp_func()
 
     Both "a"s are subject to equal fields propagation and can be replaced by:
     - Item_field::propagate_equal_fields(ANY_SUBST) for the left "a"