Add accessor methods for classes in item_timefunc.h

class Item_char_typecast :public Item_str_func

...

uint get_cast_length() { return cast_length; }

class Item_func_add_time :public Item_temporal_hybrid_func

...

int get_sign() { return sign; }

class Item_func_timestamp_diff :public Item_int_func

...

const interval_type get_int_type() { return int_type; };