[libclang] Enhance clang_getOverriddenCursors.

[libclang] Enhance clang_getOverriddenCursors.

Basically the current design is:
-for an implementation method, show as overridden the interface method.
  This is not useful, and is inconsistent with the C++ side
-for an interface method, show as overridden the protocols methods (this is desirable)
  and the methods from the categories; methods from categories are not useful
  since they are considered the same method (same USR).
-If there is a protocol method or category method reported, it does not check the
  super class for overridden methods. This is really problematic since
  overridden methods from super class is what we want to give back.

Change clang_getOverriddenCursors to show as overridden any method in the class's
base class, its protocols, or its categories' protocols, that has the same
selector and is of the same kind (class or instance).
If no such method exists, the search continues to the class's superclass,
its protocols, and its categories, and so on. A method from an Objective-C
implementation is considered to override the same methods as its
corresponding method in the interface.

rdar://10967206

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152270 91177308-0d34-0410-b5e6-96231b3b80d8