[analyzer] Cluster bindings in RegionStore by base region.

This should speed up activities that need to access bindings by cluster,
such as invalidation and dead-bindings cleaning. In some cases all we save
is the cost of building the region cluster map, but other times we can
actually avoid traversing the rest of the store.

In casual testing, this produced a speedup of nearly 10% analyzing SQLite,
with /less/ memory used.

llvm-svn: 161636