[serialization] Add the HasBuiltinModuleAccess hack to fallback modules too.

This only affects people with SWIFT_USE_LIBRARY_MODULES turned on, i.e. me.

Swift SVN r5956

lib/Serialization/SerializedModuleLoader.cpp

@@ -100,6 +100,9 @@ static Module *makeTU(ASTContext &ctx, AccessPathElem moduleID,
   TranslationUnit *TU = new (ctx) TranslationUnit(moduleID.first, comp, ctx,
                                                   TranslationUnit::Library);
 
+  TU->HasBuiltinModuleAccess = (moduleID.first.str() == "swift");
+  performAutoImport(TU);
+
   ctx.LoadedModules[moduleID.first.str()] = TU;
 
   std::vector<unsigned> BufferIDs;