Summary:
To find a method in non-abstract sub-classes, this diff applies the
same heuristics of inferbo.
* If the class is an interface: Find its unique sub-class and apply the heuristics recursively.
* If the class is an abstract class: Find/use its own summary if possible. If not found, find
one (arbitrary but deterministic) summary from its sub-classes.
* Otherwise: Find its own summary.
Reviewed By: ezgicicek
Differential Revision: D20647101
fbshipit-source-id: 2f8f3ff81
Sungkeun Cho