[IRGen] When creating a module object, mark __Swift_AST as used.

Without doing this, `__Swift_AST` gets stripped from the output.
We also need to call `IGM.finalize()` before `::performLLVM()` in the
`createSwiftModuleObjectFile()` function, so that we update the
section to mark it as retained.

rdar://123504095

Author: al45tair

lib/IRGen/IRGen.cpp

@@ -1655,8 +1655,10 @@ void swift::createSwiftModuleObjectFile(SILModule &SILMod, StringRef Buffer,
     break;
   }
   }
+  IGM.addUsedGlobal(ASTSym);
   ASTSym->setSection(Section);
   ASTSym->setAlignment(llvm::MaybeAlign(serialization::SWIFTMODULE_ALIGNMENT));
+  IGM.finalize();
   ::performLLVM(Opts, Ctx.Diags, nullptr, nullptr, IGM.getModule(),
                 IGM.TargetMachine.get(),
                 OutputPath, Ctx.getOutputBackend(), Ctx.Stats);