code: protected TypeMirror interfaceGenericTypeFor(TypeElement element, Class interfaceType) {
          return interfaceGenericTypeFor(element, interfaceType.getName());
      }
docs: Finds the generic type for the given interface for the given class element.
  <p>
  For example, for <code>class AProvider implements Provider</code>
  element = AProvider
  interfaceType = interface javax.inject.Provider.class
  return A
  
  @param element       The class element
  @param interfaceType The interface
  @return The generic type or null