Print diagnostic groups as part of the LLVM printer in the same manner as the
Swift one does, always. Make `-print-diagnostic-groups` an inert option, since we
always print diagnostic group names with the `[#GroupName]` syntax.
As part of this, we no longer render the diagnostic group name as part
of the diagnostic *text*, instead leaving it up to the diagnostic
renderer to handle the category appropriately. Update all of the tests
that were depending on `-print-diagnostic-groups` putting it into the
text to instead use the `{{documentation-file=<file name>}}`
diagnostic verification syntax.
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
