[libclang] For a class extension, give it a unique USR but for any property or ivar

[libclang] For a class extension, give it a unique USR but for any property or ivar
it contains give it a USR based on its semantic context, which is the interface.

This follows what we already did for objc methods. rdar://10371669

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143464 91177308-0d34-0410-b5e6-96231b3b80d8