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

[Erick Velez via cfe-commits]

evelez7 wrote:

If we're open to adding a flag in the base `Info` like `IsClassSpecialization`, then we can probably easily deal with these by trying to reconstruct the specialization's arguments (will be something like "Foo<T, int>.json"). Functions don't produce their own files so function specializations wont be a problem. Then we can keep this layout. @ilovepi 

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