CF_RETURNS_[NOT_]RETAINED on a param makes the inner pointer __nullable.

That is,

  void cf2(CFTypeRef * __nullable p CF_RETURNS_NOT_RETAINED);

is equivalent to

  void cf2(CFTypeRef __nullable * __nullable p CF_RETURNS_NOT_RETAINED);

More rdar://problem/18742441

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240186 91177308-0d34-0410-b5e6-96231b3b80d8