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

[Paul Kirth via cfe-commits]

ilovepi wrote:

>  Right now, since I just ripped the Mustache generator code, it creates folders for each namespace and emits the nested entities there. This actually represents a problem for template specializations because the code tries to write another JSON object (the specialization) to the same file as the base template, which is invalid.
 
After re-reading this, I have a vague memory that this used to happen for everything, and it somehow got fixed for YAML. Probably back when Brett was working on this.

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