[SwiftSyntax] Refactor AbsolutePosition
AbsolutePosition being a mutable reference type easily leads to bugs
where an AbsolutePosition is modified. Making it immutable eliminates
this issue. Furthermore, the introduction of SourceLength should allow
easier manipulation of AbsolutePositions on the client side.
We still cannot make AbsolutePosition a value type since it is used
inside AtomicCache, but the immutability gives the same safety.
