i believe you can only foreshorten a radius or diameter dimension.
As Kenneth said, I believe you can only foreshorten radius or diameter dimensions, and even at that, the workflow to actually get it to work is a pain. You can always cheat it by moving the dimension close to one side and hiding the dimension line and leader line you don't want showing. Right clicking on the dimension line and leader line will give you the below option:
Which would leave your dimension looking like this:
This doesn't give you the usual indicators of a foreshortened dimension, like the double arrow in the shortened direction, but it's what I usually go with in the small handful of situations it's needed.
If you can wait a few months, I believe this is a feature that is going to be a part of 2016 though.