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

[via llvm-commits]

tcottin wrote:

Yes, as I wrote, this is the expected output for this PR.

Before rebasing the next patch I would like to get this PR reviewed.
If there is something bigger we need to change, I think it is easier to first change it for this PR only and then rebase the last patch.

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