[clang-tools-extra] [clang-doc] add a JSON generator (PR #142483)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi approved this pull request.

I think this is a good start, modulo a couple fixes on reserving space in json::Arrays. I'm fine if we keep incrementally adding testing rather than doing it all up front. When we're done I imagine most of the YAML tests can be converted, and we can just drop those bits.

https://github.com/llvm/llvm-project/pull/142483