Followup work of #140498 to continue the work on clangd/clangd#529
Introduce the use of the Clang doxygen parser to parse the documentation
of hovered code.
- ASTContext independent doxygen parsing
- Parsing doxygen commands to markdown for hover information
Note: after this PR I have planned another patch to rearrange the
information shown in the hover info.
This PR is just for the basic introduction of doxygen parsing for hover
information.
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Co-authored-by: Maksim Ivanov <emaxx@google.com>
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