Remove CXCursorSet and related APIs.  There are no known clients.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180174 91177308-0d34-0410-b5e6-96231b3b80d8
Add test case for -Wdeprecated-objc-pointer-introspection, and
tweak warning to suggest that it is just a bad thing to do.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180175 91177308-0d34-0410-b5e6-96231b3b80d8