Add a heuristic to the dead stores checker to prune dead stores for variables annotat...

Add a heuristic to the dead stores checker to prune dead stores for variables annotated with '__block'.  This is overly conservative, but now the analyzer doesn't report dead stores for variables that can be updated by a block call.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90364 91177308-0d34-0410-b5e6-96231b3b80d8