[Attributor] Look through allocated heap memory

AAPointerInfo, and thereby other places, can look already through
internal global and stack memory. This patch enables them to look
through heap memory returned by functions with a `noalias` return.

In the future we can look through `noalias` arguments as well but that
will require AAIsDead to learn that such memory can be inspected by the
caller later on. We also need teach AAPointerInfo about dominance to
actually deal with memory that might not be `null` or `undef`
initialized. D106397 is a first step in that direction already.

Reviewed By: kuter

Differential Revision: https://reviews.llvm.org/D109170