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

[via llvm-commits]

mccakit 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?

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