[clang-tools-extra] [llvm] [llvm] add support for mustache templating language (PR #105893)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi requested changes to this pull request.

I think there is still quite a lot to do here ATM. First, I see many things marked as resolved, that weren't actually changed. Unless there's a good reason I think those should be addressed.

In particular, I and the other reviewers have pointed out that we should be using `std::string` over `SmallString<0>`. It's my fault for suggesting that in the first place, so sorry I've made extra work for you.

Additionally, I think there's a few opportunities w/ the JSON bits to just use what's in the Support library already, without you reimplementing those APIs.

Lastly, I think we should probably either be leaning on the output streams more for rendering/output buffers, or we should just be returning values from those APIs directly. 

https://github.com/llvm/llvm-project/pull/105893