Propagate '-application-extension' to module interface loader sub-invocations.

This flag restricts availability of certain symbols to ensure the code cannot use declarations that are explicitly unavalable to extensions. This restriction should be passed down to dependency modules also.

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1559,6 +1559,11 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
     genericSubInvocation.getFrontendOptions().DisableImplicitModules = true;
     GenericArgs.push_back("-disable-implicit-swift-modules");
   }
+  // If building an application extension, make sure API use
+  // is restricted accordingly in downstream dependnecies.
+  if (langOpts.EnableAppExtensionRestrictions) {
+    GenericArgs.push_back("-application-extension");
+  }
   // Save the parent invocation's Target Triple
   ParentInvocationTarget = langOpts.Target;