Make FallThrough resilient to missing source information.

[pkwarren]

Update the FallThrough check to gracefully handle missing source code
information, which occurs when using certain Lombok annotations. This is
similar to the existing workaround added to the ParameterName bug
pattern.

Fixes #613.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[googlebot]

CLAs look good, thanks!

[pkwarren]

@cushon / @ronshapiro - Are you able to review this PR?

[cushon]

Do you have a test? Have you seen this happen in builds that don't use lombok?

[pkwarren]

This only occurs in builds that use Lombok from my experience. This is the same change made in #1135 just in another check we've found which fails without this information.

[cushon]

Did you see examples where next.getStartPosition() wasn't available, or just caseEndPosition(state, caseTree) ?

[cushon]

I think this is subsumed by #1573.

[cushon]

Closing since I think this will be fixed by #1573.