MDEV-16991 will integrate nanosecond rounding functionality as methods into class Temporal (and its descendants Time, Datetime and Temporal_hybrid). There will be no rounding functionality for pure MYSQL_TIME.

To reuse rounding easier in these functions:

• str_to_datetime_with_warn()
• double_to_datetime_with_warn()
• decimal_to_datetime_with_warn()
  we need to remove direct calls for functions like str_to_datetime(), which operate on pure MYSQL_TIME, and add Temporal_hybrid constructors instead. So nanoseconds rounding will automatically start working in xxx_to_datetime_with_warn() as soon as it's implemented in Temporal_hybrid constructors.

A constructor making a Temporal_hybrid from strings already exists and is used in Type_handler_temporal_with_date::create_literal_item() and Type_handler_date_common::create_literal_item(). str_to_datetime_with_warn() will reuse it.

A new constructor making a Temporal_hybrid from numbers (namely from Sec6) will be added under terms of this task.
Functions double_to_datetime_with_warn(), decimal_to_datetime_with_warn(), int_to_datetime_with_warn() will reuse this new constructor.

This is a self-containing dependency change to make the future patch for MDEV-16991 as simple and clear as possible.