When checking availability attributes for consistency between an
overriding and overridden method, allow the overridden method to have
a narrower contract (introduced earlier, deprecated/obsoleted later)
than the overriding method. Fixes <rdar://problem/12992023>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172567 91177308-0d34-0410-b5e6-96231b3b80d8