[clang] [clang][ssaf] Implement JSONFormat (PR #180021)

[Balázs Benics via cfe-commits]

https://github.com/steakhal commented:

It's getting impeachable. Rock solid already but I had a few last remarks.
I also went over the tests and to my eyes, they look good.
I also feel like we manually "built" a testing corpus for fuzzing the readSummary API.
Maybe we should actually do that in the future.

https://github.com/llvm/llvm-project/pull/180021