There's a bit of confusion here worth clearing up: a SILModule contains "all the SIL you are working with", not "all the SIL from a particular module". (It's closer to ASTContext than ModuleDecl in that way, but LLVM has both LLVMContext and llvm::Module, and a SILModule definitely depends on ASTContext, so…)
It'd be easy enough to put the owning SerializedASTFile into SILFunction, perhaps instead of or inside the DebugScope. But the information isn't there now.