Added GRStateManager::scanReachableSymbols(), a method which scans the reachable

symbols from an SVal.

- Fixed a bug in EnvironmentManager::RemoveDeadBindings() where it did not mark
  live all the symbols reachable from a live block-level expression.

- Fixed a bug in the retain/release checker where it did not stop tracking
  symbols that 'escaped' via compound literals being assigned to something the
  BasicStoreManager didn't reason about.

llvm-svn: 64534