In order to format comments, it's helpful if they're tracked. This
tracks them separately from tokens in order to avoid interfering with
parse; it'd be inconvenient if comment tokens could show up in arbitrary
locations, albeit possible to support.
This additionally extracts out the TokenIterator support into a template
in order to generally have it available for IndexBase types. I'm only
adding it for CommentInfo, not sure if we'll want it elsewhere, but this
structure still felt like a good fit.
Since formatting covers comments, and the CHECK lines are in comments,
it can create recursive behaviors. This introduces AUTOUPDATE-SPLIT as a
way of formally designating a split to exclusively be used for
autoupdate output.
This extracts out the SourceBuffer handling of `-` in order to trivially
share it.
Note this still has a number of TODOs, it's just setting up the
essential subcommand infrastructure, with some tests demonstrating that
it at least does something.
Jon Ross-Perkins