The class `KnownSVal` was very magical abstract class within the `SVal`
class hierarchy: with a hacky `classof` method it acted as if it was the
common ancestor of the classes `UndefinedSVal` and `DefinedSVal`.
However, it was only used in two `getAs<KnownSVal>()` calls and the
signatures of two methods, which does not "pay for" its weird behavior,
so I created this commit that removes it and replaces its use with more
straightforward solutions.
163301d785