[C++][Dataset] Refactor Expression::Assume to return a Result

Expression::Assume can abort if the two expressions are not valid against a single schema. This is not ideal since a schema is not always easily available. The method should be able to fail gracefully in the case of a best-effort simplification where validation against a schema is not desired.

https://github.com/apache/arrow/pull/8037#discussion_r475594117