parser: restoring from ast to string: RestoreStringWithoutDefaultCharset

现象

create table b.t1 (val timestamp default '0000-00-00 00:00:00')
-- was re-written to
CREATE TABLE `hello`.`t1` (`val` TIMESTAMP DEFAULT _UTF8MB4'0000-00-00 00:00:00')

添加RestoreStringWithoutDefaultCharset flag后, 不会给 utf8mb4 string添加前缀.

问题:

• string literal的charset, 可显式指定, 如 _latin1'hello'
• 也可由ParseOneStmt()的charset参数决定
• 但tidb/parser:ValueExpr.Restore()中, 只和UTF8MB4比较
• 如何理解?