[clang][dataflow] Generalize custom comparison to return tri-value result.

Currently, the API for a model's custom value comparison returns a
boolean. Therefore, models cannot distinguish between situations where the
values are recognized by the model and different and those where the values are
just not recognized.  This patch changes the return value to a tri-valued enum,
allowing models to express "don't know".

This patch is essentially a NFC -- no practical differences result from this
change in this patch. But, it prepares for future patches (particularly,
upcoming patches for widening) which will take advantage of the new flexibility.

Differential Revision: https://reviews.llvm.org/D137334