Using Clang parser to generate AST in memory

I need to use the Clang parser to parse C++ source code, in this case to AST rather than IR; that is, I’m looking to supply filenames and get back syntax trees as in-memory data structures. What’s the best way to do this? I got as far as tracing the driver code to FinalPhase = phases::Compile; which looks like what I need to do, but I can’t see what to call to get back the AST?

Docs for libtooling are here:
http://clang.llvm.org/docs/LibTooling.html