[Objective-C]. Modern property getters have side-effects.
So, place warning about property getter should not be used for side-effect
under its own group so warning can be turned off.
rdar://19137815


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224479 91177308-0d34-0410-b5e6-96231b3b80d8