Fix PR 4594 by refactoring almost all casting logic from GRExprEngine::VisitCast

Fix PR 4594 by refactoring almost all casting logic from GRExprEngine::VisitCast
to SValuator::EvalCast. In the process, the StoreManagers now use this new cast
machinery, and the hack in GRExprEngine::EvalBind to handle implicit casts
involving OSAtomicCompareAndSwap and friends has been removed (and replaced with
logic closer to the logic specific to those functions).

llvm-svn: 76641