Even if the design docs would exist, it would take months to read them, without any guarantee that they correspond to the reality.
Meanwhile, automated analysis of actual code can give you at least high-level overview of the codebase and maybe a hint where to start digging. Getting AST is a first step required for most automated tools to do their work.
EDIT: I acted too rashly and downvoted your post before I realized what are we really talking about. Sorry about this. I still am convinced that automatic, static analysis of the code is the way to go, but you obviously don't deserve a downvote for having different opinion. I'll try to make it up to you by being more careful in the future :)
Static Analysis would be my second step, but first I'd have a look at the architectural documentation. I can't imagine that a project of this size wouldn't at least have a Powerpoint explaining the structure and concepts of the code.
Your question sounds like something that someone with either no real experience and/or no experience of an object-oriented language would ask.
100 million lines is a lot of code. Why do you need to "parse it to extra the AST"? That's crazy.
Do you have the original design documents and architectural documentation? If you do, read it.