Plan out testing clang internal features with new Checked C features

[dtarditi]

We need to test printing, AST serialization, AST transformation, and AST serialization with the new Checked C features. This work item is to understand the tests that exist for these features and how to extend them to Checked C features.

[dtarditi]

No need to have a separate planning work item for this. We have issues open for the features that remain to be tested better. Closing this issue.