[clang] [clang-tools-extra] [llvm] [clangd][WIP] Add doxygen parsing using standalone doxygen parser (PR #128591)

[via llvm-commits]

tcottin wrote:

After some more discussions and suggestions in clangd/clangd#529, I made additional changes.
Now I think this patch has gotten too big, so I will split this into multiple smaller patches starting with #140498

https://github.com/llvm/llvm-project/pull/128591