[Attributor] Remove capture tracker usage and follow uses explicitly

Before we used the capture tracker to follow pointer uses, now we do it
explicitly ourselves through the Attributor API. There are multiple
benefits: For one, the boilerplate is cut down by a lot. The class,
potential copies vector, etc. is all not needed anymore. We also do
avoid explicitly looking through memory here, something that was
duplicated and should only live in the `checkForAllUses~ helper. More
importantly, as we do simplifications we need to make sure all parties
are in sync when they reason about uses. The old way did not allow us to
do this but the new one does as every use visiting AA goes through
`checkForAllUses` now..