A cleanup for MDEV-17317 Add THD* parameter into Item::get_date() and stricter data type control to "fuzzydate"

Fixing C++ function check_date() to get the "fuzzydate" as
date_mode_t rather than ulonglong, so conversion from
date_time_t to ulonglong is now done inside C++ check_date(),
and no conversion is needed in the callers' code.

As an additional safety, modified the code not to pass
TIME_FUZZY_DATE to the low level C functions:
- check_date()
- str_to_datetime()
- str_to_time()
- number_to_datetime()
because TIME_FUZZY_DATE is known only on the C++ level,
C functions do not know it.

Soon we'll be adding more flags into the C++ level (i.e. to date_time_t),
e.g. for rounding. It's a good idea to prevent passing C++ specific
flags into pure C routines before this change.

Asserts were added into the affected C functions to verify
that the caller passed only known C level flags.

sql/sql_type.cc

@@ -480,7 +480,8 @@ void Temporal_with_date::make_from_item(THD *thd, Item *item)
 
 void Temporal_with_date::check_date_or_invalidate(int *warn, date_mode_t flags)
 {
-  if (check_date(this, pack_time(this) != 0, ulonglong(flags), warn))
+  if (check_date(this, pack_time(this) != 0,
+                 ulonglong(flags & TIME_MODE_FOR_XXX_TO_DATE), warn))
     time_type= MYSQL_TIMESTAMP_NONE;
 }