Emit the category documentation URL into serialized diagnostics as part of
the existing RECORD_CATEGORY, using the form
<category name>@<category URL>
and keeping the existing "category name length" field referring to the
length of the category name itself (up to the @). There is a corresponding
update to libclang to process such category names correctly and add
API, but it isn't strictly necessary: readers that use the category
name length field correctly will see no behavior change, whereas
readers that ignore it will merely see the extra `@<category URL>`.
This is a step toward staging out our (mis)use of the "flags" field as
the place to stash educational note and diagnostic group documentation
URLs.
The serialized diagnostic format has some extra fields that we can
adopt for diagnostic groups. Specifically:
* Category: store the diagnostic group name here
* Flags: extend the hack used by educational notes of placing Markdown file paths here
