[analyzer] DeallocChecker: Don't warn on release of readonly assign property in dealloc.

[analyzer] DeallocChecker: Don't warn on release of readonly assign property in dealloc.

It is common for the ivars for read-only assign properties to always be stored retained,
so don't warn for a release in dealloc for the ivar backing these properties.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259998 91177308-0d34-0410-b5e6-96231b3b80d8