scope for super-classes of XClass should not contain DataTypes

[meysholdt]

Tests are at:

https://github.com/merks/Xcore/blob/master/org.eclipse.emf.ecore.xcore.tests/src/org/eclipse/emf/ecore/xcore/tests/scoping/ClassExtends.xcore

They're currently disabled. (remove the "__" prefix to enable them).