[analyzer] Introduce a new interface for tracking

Tracking values through expressions and the stores is fundamental
for producing clear diagnostics.  However, the main components
participating in this process, namely `trackExpressionValue` and
`FindLastStoreBRVisitor`, became pretty bloated.  They have an
interesting dynamic between them (and some other visitors) that
one might call a "chain reaction". `trackExpressionValue` adds
`FindLastStoreBRVisitor`, and the latter calls `trackExpressionValue`.

Because of this design, individual checkers couldn't affect what's
going to happen somewhere in the middle of that chain.  Whether they
want to produce a more informative note or keep the overall tracking
going by utilizing some of the domain expertise.  This all lead to two
biggest problems that I see:

  * Some checkers don't use it
  This should probably never be the case for path-sensitive checks.

  * Some checkers incorporated their logic directly into those
    components
  This doesn't make the maintenance easier, breaks multiple
  architecture principles, and makes the code harder to read adn
  understand, thus, increasing the probability of the first case.

This commit introduces a prototype for a new interface that will be
responsible for tracking.  My main idea here was to make operations
that I want have as a checker developer easy to implement and hook
directly into the tracking process.

Differential Revision: https://reviews.llvm.org/D103605