Fix Objective-C metadata for properties from class extensions after r251874

Fix Objective-C metadata for properties from class extensions after r251874

After, properties from class extensions no longer show up in
ObjCInterfaceDecl::properties().  Make ObjCCommonMac::EmitPropertyList()
explicitly look for properties in class extensions before looking at
direct properties.

Also add a test that passes both with clang before r251874 and after this
patch (but fails with r251874 and without this patch).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254622 91177308-0d34-0410-b5e6-96231b3b80d8