On a 2017 LaCrosse getting into the trunk to access a dead or disconnected battery is more complicated than you guys think.
The battery for a 2017 is in the trunk not in the engine compartment, and TTT says he has it disconnected.
There is no easy way to open the trunk lid from outside without power.
On a 2017 the fold down seat backs are released only from inside the trunk not from inside the car like the earlier models.
There is a manual emergency trunk release inside the trunk but you cant get to it unless you are already in the trunk.
The only solution I see is to use the manual key to get in the car if it is locked, then open the hood and connect a good battery to the remote terminals in the engine compartment (as 1arnold1 suggested) to enable opening the trunk. Connect the battery that is in the trunk to start the car or jump start it if the battery is dead.
I'm curious if TTT solved his problem in a different way.