- constify some uses of MemRegion* (MemRegion should be immutable).

- Added new region "SymbolicRegion", which maps symbol values to the region domain.
- Enhanced BasicStore::getFieldLValue() to return a FieldRegion (using SymbolicRegion)
- Added some utility methods to GRState for fetch svals from the store.
- Fixed regression in CheckNSError (we weren't getting the value bound to the parameter)

llvm-svn: 57717