[Frontend] Allow -typecheck-module-from-interface to emit no outputs

-typecheck-module-from-interface doesn't emit textual output as
configured in frontend option. The real output is diagnostics but that
is not tracked by output computer which is using the option.

Teach `swift-frontend` that it doesn't actually emit any real output and
tell serialization not to emit swiftmodule.

lib/Frontend/Serialization.cpp

@@ -65,8 +65,10 @@ void swift::serializeToBuffers(
     std::unique_ptr<llvm::MemoryBuffer> *moduleDocBuffer,
     std::unique_ptr<llvm::MemoryBuffer> *moduleSourceInfoBuffer,
     const SILModule *M) {
+  // Serialization output is disabled.
+  if (options.OutputPath.empty())
+    return;
 
-  assert(!options.OutputPath.empty());
   {
     FrontendStatsTracer tracer(getContext(DC).Stats,
                                "Serialization, swiftmodule, to buffer");