Change RegionStore to always use ImmutableMapRef for processing cluster bindings.

This reduces analysis time by 1.2% on one test case (Objective-C), but
also cleans up some of the code conceptually as well.  We can possible
just make RegionBindingsRef -> RegionBindings, but I wanted to stage
things.

After this, we should revisit Jordan's optimization of not canonicalizing
the immutable AVL trees for the cluster bindings as well.

llvm-svn: 169571