[clang] [clang-tools-extra] [llvm] [clangd] introduce doxygen parser (PR #150790)

[via llvm-commits]

tcottin wrote:

> Hey, I merged this PR into main and build it, but doxygen comments don't work. Am I supposed to tweak something in my .clangd config? Or do you intend on enabling doxygen hower on the third patch?

Yes, you need to enable this in your `.clangd` config:

```yaml
Documentation:
  CommentFormat: Doxygen
```

This PR is parsing doxygen commands and highlights them as bold text.
Additionally, some inline commands like `\p` will render their argument according to their render kind, e.g. monospaced for `\p`.
Also hovering over a documented parameter shows the parameter documentation.

The next PR will additionally rearrange and highlight commands like `\brief`, `\return` etc differently in the hover.

https://github.com/llvm/llvm-project/pull/150790