[analyzer] RetainCountChecker: CF properties are always manually retain-counted.

In theory we could assume a CF property is stored at +0 if there's not a custom
setter, but that's not really worth the complexity. What we do know is that a
CF property can't have ownership attributes, and so we shouldn't assume anything
about the ownership of the ivar.

rdar://problem/20076963

llvm-svn: 231553