Remove 'SelfRegion' field from both BasicStoreManager and RegionStoreManager.

SelfRegion represented the object bound to 'self' (when analyzing Objective-C
methods) upon entry to a method. Having this region stored on the side ignores
the current stack frame that we might be analyzing (among other things), and is
a problem for interprocedural analysis.

For RegionStoreManager, the value for SelfRegion is just lazily created.

For BasicStoreManager, the value for SelfRegion is bound eagerly to 'self', but
no explicit tracking of SelfRegion on the side is made.

As part of this change, remove the restriction in BasicStoreManager that we only
track ivars for 'self'. This shouldn't actually change anything in terms of
precision, and simplifies the logic.

llvm-svn: 79694