[clang-tools-extra] [clang-doc] Use SmartMutex when visiting the AST (PR #135514)

[Paul Kirth via cfe-commits]

ilovepi wrote:

### Merge activity

* **Apr 14, 8:26 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/135514).


https://github.com/llvm/llvm-project/pull/135514