Hey Kazutaka,
Thank you for being interested in this project!
It largely depends on what you currently know about Swift compiler project.
Have you already checked out and built the compiler?
Have you already made any contribution to the project?
Have you already tried latest libSyntax APIs?
If you haven't done anything above, I recommend you to start them now.
Fixing StarterBugs on JIRA is a good start point. Experiencing it would be a good way to you to getting some ideas about the project.
As a side note, Here's a related discussion about this specific task: