build a last-writer shadow-memory-based tool

I'm filing this here as it would use Umbra and thus be most easily added to the DrMem repository.

This has been on our list of nice finished tools to add to the release package for a while. Pasting the description from http://drmemory.org/projects.html:

One tool is a last-writer tool, similar to what is described in the paper "Data Provenance Tracking for Concurrent Programs" in CGO 2015 by Brandon Lucia and Luis Ceze. Knowing which thread and which instruction last wrote a memory value can be invaluable during debugging. This tool would use simple instrumentation to record that information.