--enforce-clean accepts a tri-state value but only has boolean behaviour

As far as I can tell, there is no code which differentiates between `AllowIgnored` and `AllowDirty`... @sitaktif do you remember any history here? AFAICT there has never been any code which differentiates between these, or any tests which set `--enforce-clean=allow-dirty` (or indeed which set `--enforce-clean=allow-ignored`)?

https://github.com/bazel-contrib/target-determinator/pull/86 is going to simplify this behaviour to be more boolean (but doesn't remove the three variants) - I _think_ this is ok (and that we should just remove the extra unused variant), but figured I should file an issue to discuss...

Review PR #86 and trace the implementation of the --enforce-clean flag, including how its three values are parsed and used. Search the repository for tests or code covering allow-dirty and allow-ignored, then compare the findings with the issue's history. Done means the supported behavior and remaining variants are consistent and covered by appropriate tests.

Written by the indexing model from the issue text.