A word of warning: I tried a cheapo USB Micro B female to USB C male adapter from Amazon and while it works at a basic level, whatever magic signaling the Nexus 5X needs to do to enable rapid charging got lost in translation no matter which USB A to USB Micro B cable I connected it to; on the lock screen, which is the only place I can find a reliable charging indication on the Nexus 5X, I never got anything other than "Charging slowly." I've ended up just using USB A to USB C cables for charging instead.

Other micro B to C adapters may do better, but I'm not holding my breath.

Hopefully some proper USB Charging Standard car adapters will come out soon (supporting the Nexus 5X/6P 3A/5V charging rate) too.