MDEV-29919: Support INSERT ... AS alias ON DUPLICATE KEY UPDATE

[MooSayed1]

MySQL 8.0.20 introduced a new syntax for INSERT ... ON DUPLICATE KEY UPDATE that allows referencing the inserted row using an alias instead of the VALUES() function:

-- Old syntax (still supported)
INSERT INTO t1 (a, b) VALUES (1, 2)
  ON DUPLICATE KEY UPDATE b = VALUES(b);

-- New syntax (this patch adds support)
INSERT INTO t1 (a, b) VALUES (1, 2) AS new
  ON DUPLICATE KEY UPDATE b = new.b;

The alias syntax is cleaner and more readable, especially when referencing multiple columns or using expressions like new.a + new.b.

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Implementation

• Parser: Added opt_values_row_alias rule to accept AS alias after VALUES
• LEX: Added insert_values_alias field to store the alias
• Name resolution: In Item_field::fix_fields(), when we're in ON DUPLICATE KEY UPDATE and the table qualifier matches the alias, we convert the reference to an Item_insert_value (equivalent to VALUES())

The traditional VALUES() function continues to work unchanged.

Tests

Added mysql-test/main/insert_update_alias.test covering:

• Basic INSERT ... AS alias ON DUPLICATE KEY UPDATE
• Multiple rows with alias
• Expressions using alias columns (new.a + new.b)
• Backward compatibility with VALUES() function
• Mix of alias and table column references
• INSERT without ON DUPLICATE KEY (alias ignored)
• Different alias names

[CLAassistant]

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