Ensure isCompound() semantics are properly spread over lang model classes

[ljnelson]

javac models intersection types and union types as ClassType subtypes named IntersectionClassType and UnionClassType respectively. Consequently, in javac code, anywhere a ClassType is handled, it could also potentially be handling either an IntersectionClassType or a UnionClassType or both.

This issue will remain open until I'm satisfied these concerns have been spread properly over the disjoint javax.lang.model.type.* classes.

[ljnelson]

This intersects with #1 because some visitors test for isCompound() while visiting ClassType. In a javax.lang.model.*-based system, these would be different visitation methods. In a visitor-less system, these need to be handled separately.