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

[Erick Velez via cfe-commits]

evelez7 wrote:

> This sounds promising. I'm fine w/ adding a field to track this. BTW, what does clang do? I'm wondering if we should track more than 1-bit of info here.

As far as I can tell, inside the AST Clang makes use of the `isa<>` mechanisms (which we could also leverage) or pointer unions like https://github.com/llvm/llvm-project/blob/f427d3aa9d43b37e6318677f96262aab73545d57/clang/include/clang/AST/DeclCXX.h#L486-L487

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