Disable relevance reduction of Closure methods and properties when object expression type is Closure

Consider the following:
```groovy
void meth(Closure closure) {
  closure.|
}
```

When invoking content assist (Ctrl+Space) at `|`, the methods and properties of groovy.lang.Closure are very low in the list of proposals.  This is due to very-well-known type adjustments made within `org.codehaus.groovy.eclipse.codeassist.proposals.AbstractGroovyProposal.getRelevanceClass()`.