[clang-tools-extra] [clang-doc] Add HTMLMustacheGenerator.cpp (PR #138060)

[Paul Kirth via cfe-commits]

ilovepi wrote:

### Merge activity

* **May 12, 9:52 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/138060).


https://github.com/llvm/llvm-project/pull/138060