This is because IntelliJ IDEA puts all of the java libs in the classpath, and for whatever reason Forge cannot read that jar file. The real fix would be for Forge to catch this error gracefully. One workaround is to make a new JDK in IDEA (CTRL+ALT+SHIFT+S, SDKs on the left, plus at the top, point to your 1.7 JDK install) and remove all of the classpath jars. Then use this alternative JRE in your run configuration.
This is from Forge making sure that net.minecraft.src packages are only inside the minecraft jar. I am assuming you are recompiling the whole Minecraft source in IDEA, so the Minecraft jar is actually your output folder. You need to reorder the dependencies of this module so that the <module source> is the first entry, not the java JDK. In any other child modules that depend on this module, you will need to make sure this module comes first on the dependency list before the child's <module source>.
