Implemented "FIXME" in ImutAVLTree: isEqual() now also compares the *data* value

and not just the key value when comparing trees. To do this we added data_type
and data_type_ref to the ImutContainerInfo trait classes. For values stored in
the tree that do not have separate key and data components, data_type is simply
a typedef of bool, and isDataEqual() always evaluates to true. This allows us to
support both ImmutableSet and ImmutableMap using the same underlying logic.

llvm-svn: 46130