[arcmt] Fix a bug where a property in a class extension, that did not exist
in the interface, got its attribute rewritten twice, resulting in
'weakweak' or 'strongstrong'.

rdar://11047179

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@153621 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed