comment parsing: Properties are considered like methods, and people 
think of them as having return values that may be computed. Don't
warn when using @return in their comment. // rdar://13189938


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176147 91177308-0d34-0410-b5e6-96231b3b80d8