* Do the same thing to the basicstore as in r84163.

* Add a load type to GRExprEngine::EvalLoad().
* When retrieve from 'theValue' of OSAtomic funcitions, use the type of the 
  region instead of the argument expression as the load type.
* Then we can convert CastRetrievedSVal to a pure assertion. In the future
  we can let all Retrieve() methods simply return SVal.

llvm-svn: 88888