[clang] [llvm] [clang][ssaf] Add CallGraph summary and extractor (PR #188753)

[Balázs Benics via cfe-commits]

steakhal wrote:

I decided to implement JSON support in a followup PR. I already have that ready downstream.
I didn't want to go overly deep in testing in case we might want to refine the summary later.
I think it's good enough to demonstrate some use-cases with what is presented here.

https://github.com/llvm/llvm-project/pull/188753