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

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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