R1:1f1c3f40755e

Allow lint messages to be rendered as Remarkup

Summary: Some linter messages, such as those produced by `ArcanistPHPCompatibilityXHPASTLinterRule`, contain backticks but are currently rendered as Remarkup literals. I think that it is generally desirable to allow lint messages to be rendered as Remarkup, although we should ideally have a way to render Remarkup for use on the command line (I actually think that this already exists, but I don't think that `arc lint` does this when rendering linter messages).