[IRGen] Allow IRGenDescriptor to hold a FileUnit

This will allow it to better interact with the
SILGen descriptor.

lib/FrontendTool/FrontendTool.cpp

@@ -1508,7 +1508,7 @@ generateIR(const IRGenOptions &IRGenOpts, const TBDGenOptions &TBDOpts,
            llvm::GlobalVariable *&HashGlobal,
            ArrayRef<std::string> parallelOutputFilenames) {
   if (auto *SF = MSF.dyn_cast<SourceFile *>()) {
-    return performIRGeneration(*SF, IRGenOpts, TBDOpts,
+    return performIRGeneration(SF, IRGenOpts, TBDOpts,
                                std::move(SM), OutputFilename, PSPs,
                                SF->getPrivateDiscriminator().str(),
                                &HashGlobal);