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

[Paul Kirth via llvm-commits]

ilovepi wrote:

Fix for c++20 in https://github.com/llvm/llvm-project/pull/143801, though I'm not aware that we support c++20 in tree at all yet.

The other bot failure is unrelated.

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