When lightning is less capabile than downstream, `USE <database>` may miss so failed with "no database selected"

[lance6716]

The source schema files may contain some unsupported feature, like unsupported chatset. When lightning failed to rewrite the statement, it directly uses the source file content. However, USE <database> is missing

tidb/br/pkg/lightning/importer/import.go

Lines 625 to 638 in c1e3dae

	func (worker *restoreSchemaWorker) addJob(sqlStr string, job *schemaJob) error {
	stmts, err := createIfNotExistsStmt(worker.parser, sqlStr, job.dbName, job.tblName)
	if err != nil {
	worker.logger.Warn("failed to rewrite statement, will use raw input instead",
	zap.String("db", job.dbName),
	zap.String("table", job.tblName),
	zap.String("statement", sqlStr),
	zap.Error(err))
	job.stmts = []string{sqlStr}
	} else {
	job.stmts = stmts
	}
	return worker.appendJob(job)
	}