{@link Location} that is chained.
{@link Locatable} forms a tree structure, where each {@link Locatable}points back to the upstream {@link Locatable}. For example, imagine {@link Locatable} X that points to a particular annotation,whose upstream is {@link Locatable} Y, which points to a particular method(on which the annotation is put), whose upstream is {@link Locatable} Z,which points to a particular class (in which the method is defined), whose upstream is {@link Locatable} W,which points to another class (which refers to the class Z), and so on.
This chain will be turned into a list when we report the error to users. This allows them to know where the error happened and why that place became relevant.
@author Kohsuke Kawaguchi