Various reasons, like embargoes on information, stuff we didn't want to wait for review on before shipping, or features that don't make sense for upstream like

Various reasons, like embargoes on information, stuff we didn't want to wait for review on before shipping, or features that don't make sense for upstream like `hipcc` which is an `nvcc` wrapper. Our goal is to get most modifications not in the third category into upstream at some point which makes the maintenance load bearable.