Preliminary reworking of value-binding logic in RegionStore:

(1) Introduce a new 'BindingKey' class to match 'BindingValue'.  This
gives us the flexibility to change the current key value from 'const
MemRegion*' to something more interesting.

(2) Rework additions/removals/lookups from the store to use new
'Remove', 'Add', 'Lookup' utility methods.

No "real" functionality change; just prep work and abstraction.

llvm-svn: 93136