Before this commit, SerializeModelPass and SerializeModelWrapperPass had
hard dependencies on the passes that load the model from LLVM-IR.
This commit makes this dependency optional. When the passes for model
serialization are executed, if they see that nobody requested to load
the model, they will not try to serialize it.
This prevents them from crashing when running in pipelines that only
contain passes that ignore the model.
This is particularly beneficial because `revng opt` automatically adds
`-serialize-model` at the end of each pipeline and, before this commit,
this meant that `revng opt` could not be used for unit-testing simple
llvm passes that do not use the model.
With this commit, `revng opt` can be used for unit-tests, even on LLVM IR
with missing model.
The previous design returned a model::Binary &, which did not allow
users writing the model to properly initialize cross references between
different parts of the model.
Alessandro Di Federico