RegionStore:

-refactor logic for retrieving bindings from VarDecls into
 RegionStoreManager::RetrieveVar()
- improve RegionStoreManager::CastRetrievedVal() and SimpleSValuate::EvalCastNL
  to better handle casts of values of the same canonical type as well as
  casts of LocAsInteger values.

llvm-svn: 76516