This is an interesting case study in development cycles. The Pixel XL from October 2016 uses the Snapdragon 82x released in July 2016 but first announced in March 2015 and it uses a kernel from 2014 which gives us a hint about when driver development started.
I hope so, but given that 3.18 was released almost three years ago I am not holding my breath.
They probably wanted to back port it to 3.18 but it was too much work. I'm pretty disappointed they didn't move the Pixel forward to a new kernel though, this seems to be one of the biggest issues with Android.