Rework RegionStoreManager's implementation of InvalidateRegions() to

not build a subregion map and instead do a single scan of the store.
This is done by building "region clusters" that represent the
collection of regions that have the same base region.  Invalidating
any region in a cluster means that they all should get invalidated.

This change brought out a point that Zhongxing mentioned to me
offline: the flattened memory binding has issues distinguishing
between direct and default bindings.  For example, setting the default
value for an entire struct is the same as binding to the first
element.  To address this problem, I moved the binding "tag" (Direct
or Default) from BindingVal to BdingKey (and removed BindingVal
entirely).  This requires us to do double lookups in some cases; and
there is still much more cleanup that can be done.

This change produced a noticeable speedup when analyzing sqlite3 (a
reduction of 4% in running time).

llvm-svn: 95193