[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