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

[Balázs Benics via cfe-commits]

steakhal wrote:

I've checked this and compiles with gcc-7. I was worried about the return expressions and NRVO. It turns out on gcc-7 it does not quite work as expected - but we are in the clear this time.

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