The file MultiRayShape.cc is in simulator_gazebo/gazebo/build/gazebo/server/physics.
Please note that the files you are seeking are not present in /opt/ros/diamondback. The debian build process removes the build subdirectory when creating the debian archives. That means you need to check out the stack gazebo_simulation from source. It is also important that you first call rosmake gazebo
to actually download and extract the gazebo sources.
To do so, the easiest way is to use rosinstall and create an overlay and then check out the source. The following steps should work.
source /opt/ros/diamondback/setup.bash
sudo apt-get install python-setuptools
sudo easy_install rosinstall
mkdir -p ~/ros-gazebo/workspace
rosinstall ~/ros-gazebo /opt/ros/diamondback ~/ros-gazebo/workspace
cd ~/ros-gazebo/workspace
svn co https://code.ros.org/svn/ros-pkg/stacks/simulator_gazebo/tags/diamondback simulator_gazebo
source ~/ros-gazebo/setup.bash
rosmake gazebo_worlds
Now you should be able to find the sources you want in ~/ros-gazebo/workspace/simulator_gazebo/gazebo/build/gazebo/server/physics