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

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PeterChou1 wrote:

> Did you consider using raw_ostream instead of String? in any case i agree that you should not use SmallString<0> over std::string (std::string will be more efficient as it always has a small buffer - not that you are likely to be able to use it in this use case)

I've refactor the PR to use raw_ostream now, let me know if what you think


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