document parsing. When a sub-class (c++ Objective-C) missing

a comment, grab the first comment found in its class
heirarchy. Also, when a category is mossing a comment,
grab comment of its primary class. // rdar://13647476


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180629 91177308-0d34-0410-b5e6-96231b3b80d8