[llvm] [llvm] Add a tool to check mustache compliance against the public spec (PR #142813)

[Paul Kirth via llvm-commits]

ilovepi wrote:

### Merge activity

* **Jun 11, 8:03 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/142813).


https://github.com/llvm/llvm-project/pull/142813