If a sub-view has already been rendered, it's helpful to re-render the
expansion site before rendering the next expansion view. Make this fact
explicit in the rendering interface, instead of hiding it behind an
awkward Optional<LineRef> parameter.
llvm-svn: 273789
This makes it easier to add renderers for new kinds of output formats.
- Define and document a pure-virtual coverage rendering interface.
- Move the text-based rendering logic into its a new file.
- Re-work the API to better reflect the presentation/formatting split.
llvm-svn: 273767
