[analyzer] Reimplement trackExpressionValue as ExpressionHandler
This commit moves trackExpressionValue into the Tracker interface
as DefaultExpressionHandler. It still can be split into smaller
handlers, but that can be a future change.
Additionally, this commit doesn't remove the original trackExpressionValue
interface, so it's not too big. One of the next commits will address it.
Differential Revision: https://reviews.llvm.org/D103616