time: insert item lost the fraction when encountering time string with timezone suffix

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

DROP TABLE IF EXISTS `testSetObjectTime`;
CREATE TABLE `testSetObjectTime` (
  `id` int(11) DEFAULT NULL,
  `d` time(6) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
INSERT INTO testSetObjectTime VALUES (1, '2020-01-01 12:00:00.123456000+05:00');
INSERT INTO testSetObjectTime VALUES (2, '2020-01-01 12:00:00.123456+05:00');
SELECT * FROM testSetObjectTime;

2. What did you expect to see? (Required)

MySQL has its warnings as Incorrect time value: '2020-01-01 12:00:00.123456+05:00' for column 'd' at row 1
So it won't recognize the timezone suffix and convert the 12:00:00.123456 to its default UTC timezone as 07:00:00.123456

But TiDB will convert the 12:00:00.123456 (from source +5:00, yes we recognized it and used it as source time zone) to its default Asia/Shanghai timezone (+8:00) as 15:00:00.123456, but unfortunately, we lost the fraction here.

+------+-----------------+
| id   | d               |
+------+-----------------+
|    1 | 15:00:00.123456 |
|    2 | 15:00:00.123456 |
+------+-----------------+

3. What did you see instead (Required)

+------+-----------------+
| id   | d               |
+------+-----------------+
|    1 | 15:00:00.000000 |
|    2 | 15:00:00.000000 |
+------+-----------------+

4. What is your TiDB version? (Required)

master