[clang-tools-extra] [clang-doc] serialize friends (PR #146165)

[Erick Velez via cfe-commits]

evelez7 wrote:

### Merge activity

* **Jun 30, 7:42 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/146165).


https://github.com/llvm/llvm-project/pull/146165