Support modify column to enum types.

[winkyao]

TiDB :

CREATE TABLE ti1(a INT NOT NULL, b INT, c INT) engine=InnoDB;
ALTER TABLE ti1 ADD COLUMN e ENUM('a', 'b') FIRST;
ALTER TABLE ti1 MODIFY COLUMN e ENUM('a', 'b', 'c');

reports Error 1105: unsupported modify column modify enum column is not supported