Rather than re-using `DiagnosticBehavior` to describe how a fix should
act, introduce `FixBehavior` to cover the differences between (e.g.)
always-as-awarning and downgrade-to-warning. While here, split the
`isWarning` predicate into two different predicates:
* `canApplySolution`: Whether we can still apply a solution when it
contains this particular fix.
* `affectsSolutionScore`: Whether
These two predicates are currently tied together, because that's the
existing behavior, but we don't necessarily want them to stay that way.
