[clang] [clang][ssaf] Add `JSONFormat` serialization support for `LUSummary` and `LUSummaryEncoding` (PR #184037)

[Balázs Benics via cfe-commits]

https://github.com/steakhal requested changes to this pull request.

I stil lbelieve that having so large PR is bad practice.  
I think there were good chuncks of this PR that just moved stuff around and/or didn't introduce new bahvior/features.  
Those parts should have been split into an NFC PR preceding this change. That would have reducedd the diff size by at least 1k - making it less intimidating.  
  
What was actually good, that it mirrors nicely the TUSummary merging parts - including the tests. That helped the review, so overall I'm not too grumpy about this, but I hope in the future we can align on this front.

https://github.com/llvm/llvm-project/pull/184037