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).

llvm-svn: 254622