Make `SynthesizedFileUnit` attached to a `SourceFile`. This seemed like the
least ad-hoc approach to avoid doing unnecessary work for other `FileUnit`s.
TBDGen: when visiting a `SourceFile`, also visit its `SynthesizedFileUnit` if
it exists.
Serialization: do not treat `SynthesizedFileUnit` declarations as xrefs when
serializing the companion `SourceFile`.
Resolves TF-1239: AutoDiff test failures.