clang parsing: find cuda specific statements (__syncthreads())


I wondered how to find CUDA specific statements (such as __syncthreads) in a
parsed kernel file. This should be possible as the ptx-backend seems to support
those. However, I couldn't get it by traversing the AST (not listed there).
I am using the clang API for parsing and transforming the code.

Any hints would be very helpful.