Enable bitcode vs include bitcode before submitting app
As you might imagine, you need both enabled in order to have your app support Bitcode recompilation in iTunes Connect. Just enabling it in Xcode simply means the Bitcode "architecture" is compiled, it doesn't necessarily mean that's sent to Apple.
Advance warning: I've done some tests on app submission with and without Bitcode, and for whatever reason submitting with Bitcode substantially slows down the time it takes for your binary to be processed so that it's ready for submission. Without Bitcode it can appear in a few minutes or up to maybe three hours; with Bitcode I've frequently had delays of 24 hours or more.