Is there a tutorial about developing source code transformations Clang plugin?

I’m newer to LLVM project. My purpose to use LLVM or Clang is developing a plugin that can automatically transform some existed new Standard C code to C89 Standard. I have read a week about Clang tutorial but I can’t find a lot of relative content about it. Could someone give me some advice?

I keep a few reference examples in clang-tutor.

type-correct by @SamuelMarks could also be helpful.