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

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PeterChou1 wrote:

> That's a fair point. For the API surface we're exposing, it may be better to make those private to the implementation, or at least 
> not part of the public header. To some extend that's dependent on how the library will be used in practice, though. @PeterChou1 > what are your thoughts here?

Yea I agree the only API for user is the Template class I can refactor the rest of classes out of the header files.

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