[clang][ExtractAPI] Refactor ExtractAPIVisitor to make it more extensible

Use CRTP to enable creating statically dispatched subclasses of
ExtractAPIVisitor.
This enables adding extension points and customising the behavior more
easily.

This is used in CXExtractAPI.cpp to create a specialized visitor for
Libclang as well as streamlining the batch implementation in ExtractAPIConsumer.cpp