How to convert XBOX 360 save to PC save?
Just from some light research around Google, this seems to be impossible to do, at least for Rise of the Tomb Raider. The 360 likely has a proprietary format that it saves in for that console, which is different than how the game would save on a PC.
This post on the Steam forums references the Xbox One version, which would likely be more in line with how a PC saves games, yet it's still not a likely possibility.