[AST] Associate the getter/setter methods to a property of a objc class extension.

[libclang] Index the getter/setter methods of a property of a objc class extension.

Fixes rdar://10907597

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151633 91177308-0d34-0410-b5e6-96231b3b80d8