Use Decl::getAvailability() rather than checking for the "unavailable"
attribute when determining whether we need to see an implementation of
a property. Fixes <rdar://problem/12958191>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171877 91177308-0d34-0410-b5e6-96231b3b80d8