Split Item_hybrid_func::fix_attributes into virtual methods in Type_handler

This task will add a new virtual method in Type_handler:

  virtual bool Item_hybrid_func_fix_attributes(THD *thd, Item_hybrid_func *func,

                                               Item **items,

                                               uint nitems) const;

and split this code in Item_hybrid_func::fix_attributes into the new method implementation in the relevant type handler implementations:

 switch (Item_hybrid_func::result_type()) {

 case STRING_RESULT:

 ..

 case DECIMAL_RESULT:

  ..

 case REAL_RESULT:

 ..

 case INT_RESULT:

 ..

 case ROW_RESULT:

 case TIME_RESULT:

 ..

 }

and also remove a special STRING_RESULT condition in Item_func_case::fix_length_and_dec(), to make the code symmetric across the data types.

Note, the THD* parameter to Type_handler::Item_hybrid_func_fix_attributes is not needed right now, but it will be needed soon:

• for MDEV-10575
• to get rid of current_thd calls inside count_string_length(), as it was planned to get rid of most current_thd calls in 10.3.