[analyzer] Revamp RegionStore to distinguish regions with symbolic offsets.

RegionStore currently uses a (Region, Offset) pair to describe the locations
of memory bindings. However, this representation breaks down when we have
regions like 'array[index]', where 'index' is unknown. We used to store this
as (SubRegion, 0); now we mark them specially as (SubRegion, SYMBOLIC).

Furthermore, ProgramState::scanReachableSymbols depended on the existence of
a sub-region map, but RegionStore's implementation doesn't provide for such
a thing. Moving the store-traversing logic of scanReachableSymbols into the
StoreManager allows us to eliminate the notion of SubRegionMap altogether.

This fixes some particularly awkward broken test cases, now in
array-struct-region.c.

llvm-svn: 161510