Implement: <rdar://problem/7185647> [RegionStore] 'self' cannot be NULL upon entry to a method

Here we implement this as a precondition within GRExprEngine, even though it is
related to how BasicStoreManager and RegionStoreManager model 'self'
differently. Putting this as a high-level precondition is more general, which is
why it isn't in RegionStore.cpp.

llvm-svn: 81378