[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

[clang][ExtractAPI] Improve tests for clang_getSymbolGraphForCursor

Adds a new mode to c-index-test that can fetch a single symbol symbol
graph for a given source location. This way we can be more precise when
writing tests for clang_getSymbolGraphForCursor.
Additionaly this makes it easier to debug the function.

Differential Revision: https://reviews.llvm.org/D146656