[IRGen] Fix SynthesizedFileUnit. (#31083)
SynthesizedFileUnits are attached to a SourceFile.
Make IRGen consistent with TBDGen: when processing a SourceFile, only process
the attached SynthesizedFile. This avoids IRGen/TBDGen inconsistencies.
Use SourceFile name in SynthesizedFileUnit::getDiscriminatorForPrivateValue.
Resolves TF-1249.
