[sanitizer] Add a ForEach callback interface for AddrHashMap.

This change adds a ForEach method to the AddrHashMap class which can
then be used to iterate over all the key value pairs in the hash map.
I intend to use this in an upcoming change to the memprof runtime.

Added a unit test to cover basic insertion and the ForEach callback.

Differential Revision: https://reviews.llvm.org/D111368