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

[Balázs Benics via cfe-commits]

https://github.com/steakhal commented:

The testing looks impressive alongside the chained failure context pieces.
I think that makes errors really actionable.
I'll admit, due to the size of the change I might have missed things, but on the grand scheme of things it looks good.

I only spot-checked the unit tests for the error handling, and those made sense.

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