My Xcode version is 6.3.2 (6D2105). Whenever I try to enter a negative value for any constraint constant in the Layout Builder, the value turns to 0. Even if the value was already negative and I've just changed the number itself, the value becomes 0.
Update: Still happens on 7.3 (7D175). Machine: El Capitan 10.11.4 (15E65), running on VMWare MacOS 10.8 host.
Uncheck "Constrain to margins" checkbox in the pin menu while adding spacing to nearest neighbour for the new constraints.