[analyzer] Don't accidentally strip off base object regions for lazy bindings.

If a base object is at a 0 offset, RegionStoreManager may find a lazy
binding for the entire object, then try to attach a FieldRegion or
grandparent CXXBaseObjectRegion on top of that (skipping the intermediate
region). We now preserve as many layers of base object regions necessary
to make the types match.

<rdar://problem/13239840>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175556 91177308-0d34-0410-b5e6-96231b3b80d8