[CodeCompletion] Ensure that ObjC root class completes instance

methods from protocols and categories as well

Code completion results for class methods already include instance methods
from Objective-C root classes. This commit ensures that the results also include
instance methods from protocols that the root class implements and root class
categories as well.

rdar://28012953

Differential Revision: https://reviews.llvm.org/D27257

llvm-svn: 292932