ObjectiveC migrator. When inferring readwrite property,
do not remove the setter if its availability differs
from availability of the getter (which is now turned into
a property). Otherwise, synthesized setter will
inherit availability of the property (which is incorrect).
// rdar://15300059


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193837 91177308-0d34-0410-b5e6-96231b3b80d8