BasicStore:

- Store bindings using a MemRegion -> SVal binding instead of VarDecl -> SVal
  binding. This mirrors some of the idea of RegionStore, but is far simpler and
  not nearly as functional.  This leads to some code simplification and
  some potential for some minor precision hacks.
  
Along the way...
- constify the use of MemRegion* in a few places
- add operator<<(llvm::raw_ostream, const MemRegion*)

llvm-svn: 66163