Add a THD* argument to Item_func_or_sum::fix_length_and_dec()

Let's add a THD * parameter to Item_func_or_sum::fix_length_and_dec().

We use current_thd frequently in fix_length_and_dec():

For certain data types, like TIME/DATETIME/DATE, THD is needed in the majority of Item_func_xxx, because they need access to thd->variables.sql_mode, for:

• MODE_TIME_ROUND_FRACTIONAL
• MODE_NO_ZERO_DATE
• MODE_NO_ZERO_IN_DATE

All hybrid type functions also need thd for the same reason (when returning TIME/DATETIME/DATE).

Also, the following sql_mode flags are tested in fix_length_and_dec():

• MODE_NO_UNSIGNED_SUBTRACTION

Many functions access THD in fix_length_and_dec() for other reasons than sql_mode:

• Item_bool_rowready_func2, directly, as well as when installing Arg_comparator
• Item_func_interval
• Item_func_between
• Item_func_nullif
• Item_func_case_searched
• Item_func_case_simple
• Item_func_decode_oracle
• Item_func_in, directly, and when installing in_vector and cmp_item_xxx()
• Item_func_regex, when installing Regexp_processor_pcre
• Item_equal
• Item_hybrid_func, in fix_attributes()
• Item_func_div
• Item_func_min_max, in fix_attributes()
• Item_func_get_user_var
• Item_func_get_system_var
• Item_func_json_array
• Item_func_sha2
• Item_singlerow_subselect
• Item_sum_avg
• Item_sum_variance
• Item_func_monthname
• Item_func_dayname
• Item_func_date_format
• Item_func_from_unixtime
• Item_char_typecast
• Item_func_time_to_sec
• Item_func_convert_tz
• Item_extract
• Item_func_timestamp
• Item_func_timediff
• Item_func_date_add_interval
• Item_func_add_time