Refine 'deprecated' checking for Objective-C classes/methods.

- If a deprecated class refers to another deprecated class, do not warn.
- @implementations of a deprecated class can refer to other deprecated things.

Fixes <rdar://problem/15407366> and <rdar://problem/15466783>.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@195259 91177308-0d34-0410-b5e6-96231b3b80d8