I am not sure if built-in indexing supports only indexing subset of the tree. YOu want to selectively index for speed and accuracy. The complete tree might contain multiple definitions of the same functions, as headers get copied, pre-processed, etc.

It supports selective indexing, you can specify paths to exclude from indexing or you might open just a single module, here is more info: https://www.sublimetext.com/docs/indexing.html