ObjectiveC. When introducing a new property declaration in 
parimary class and in mrr mode, assume property's default
memory attribute (assign) and to prevent a bogus warning.
// rdar://15859862


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200238 91177308-0d34-0410-b5e6-96231b3b80d8