[clang] [clang][ssaf] Improve serialization for Model types and error messages in `JSONFormat` (PR #183390)

[Balázs Benics via cfe-commits]

https://github.com/steakhal commented:

I have a weak rejection on this one.
There are parts that are NFC, and some others like changing the error messages and stuff like that are non-NFC.

I'd much prefer having these merged separately.

https://github.com/llvm/llvm-project/pull/183390