[clang-doc] Extract Info into JSON values (#138063)
Split from #133161. This patch provides the implementation of a number
of extractValue overloads used with the different types of Info.
The new helper functions extract the relevant information from the
different *Infos and inserts them into the correct fields of the JSON
values that will be used with the specific Mustache templates, which
will land separately.
Co-authored-by: Peter Chou <peter.chou@mail.utoronto.ca>
