[clangd][WIP] Add doxygen parsing for Hover

[tcottin]

With this PR I try to revive clangd/clangd#529.

I applied this patch and rebased it to main.

Note: The original author of the patch is @tom-anders and there is this abandoned phabricator review.

In addition to applying the patch, I fixed the parsing of block commands with arguments to solve one of the open points of the issue.
I also changed the checks for specific commands from name to using the comments::CommandInfo to check whether a command is a brief or return command.
This allows to handle all brief (e.g. \brief, \short) and return (e.g. \return, \returns) the same without the need to check for all the command names individually.

There was a merge conflict with #67802 which I solved without any failing tests but I am not sure yet if this is really correct.

According to @aaronliu0130 we also need to consider #78491 for this change which I did not do yet.

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[tcottin]

closing this in favor of #128591