Switch RegionStore over to using <BaseRegion+raw offset> to store

value bindings.  Along with a small change to OSAtomicChecker, this
resolves <rdar://problem/7527292> and resolves some long-standing
issues with how values can be bound to the same physical address by
not have the same "key".  This change is only a beginning; logically
RegionStore needs to better handle loads from addresses where the
stored value is larger/smaller/different type than the loaded value.
We handle these cases in an approximate fashion now (via
CastRetrievedVal and help in SimpleSValuator), but it could be made
much smarter.

llvm-svn: 93137