SI-7367 scaladoc crash on constructing the model for annotations.

Scaladoc only checks primary constructor when building annotation
model.

Here we instead find the constructor matching the annotation's symbol.
Also change TreeFactory.makeTree to return TreeEntity rather than
Option[TreeEntity] and force the caller check for EmptyTree.
Merge pull request #2392 from vigdorchik/ticket/si-7367

SI-7367 scaladoc crash on constructing the model for annotations.